Determine the file type using C#

Question

I have a web page that has the file upload component to upload the allowed document types only to my system (images only). I don't want to use the regular expression to determine the file type and using the extension.

Please let me know if there is any other means of determining file types in C#.

Answer 1

I've just converted my VB.NET class to C# to detect mime types. They will be identified by a mix of sniffing the first 256 Bytes of a file(urlmon.dll - f.e. used by internet explorer) and a set of known types.

You can also use it to check if the file is what it pretends to be(if the extension matches the type).

// check whether or not the uploaded file is an image: var contentType = MimeTypes.GetContentType(FileUpload1.PostedFile.FileName); if(contentType.StartsWith("image")) {     // do something with the image ... } 

Here's the class:

using System; using System.Collections.Generic; using System.Linq; using System.Runtime.InteropServices; using System.IO;  public class MimeTypes {     private static List<string> knownTypes;      private static Dictionary<string, string> mimeTypes;      [DllImport("urlmon.dll", CharSet = CharSet.Auto)]     private static extern UInt32 FindMimeFromData(         UInt32 pBC, [MarshalAs(UnmanagedType.LPStr)]         string pwzUrl, [MarshalAs(UnmanagedType.LPArray)]         byte[] pBuffer, UInt32 cbSize, [MarshalAs(UnmanagedType.LPStr)]         string pwzMimeProposed, UInt32 dwMimeFlags, ref UInt32 ppwzMimeOut, UInt32 dwReserverd     );      public static string GetContentType(string fileName)     {         if (knownTypes == null || mimeTypes == null)             InitializeMimeTypeLists();         string contentType = "";         string extension = System.IO.Path.GetExtension(fileName).Replace(".", "").ToLower();         mimeTypes.TryGetValue(extension, out contentType);         if (string.IsNullOrEmpty(contentType) || knownTypes.Contains(contentType))         {             string headerType = ScanFileForMimeType(fileName);             if (headerType != "application/octet-stream" || string.IsNullOrEmpty(contentType))                 contentType = headerType;         }         return contentType;     }      private static string ScanFileForMimeType(string fileName)     {         try         {             byte[] buffer = new byte[256];             using (FileStream fs = new FileStream(fileName, FileMode.Open))             {                 int readLength = Convert.ToInt32(Math.Min(256, fs.Length));                 fs.Read(buffer, 0, readLength);             }              UInt32 mimeType = default(UInt32);             FindMimeFromData(0, null, buffer, 256, null, 0, ref mimeType, 0);             IntPtr mimeTypePtr = new IntPtr(mimeType);             string mime = Marshal.PtrToStringUni(mimeTypePtr);             Marshal.FreeCoTaskMem(mimeTypePtr);             if (string.IsNullOrEmpty(mime))                 mime = "application/octet-stream";             return mime;         } catch (Exception ex)         {             return "application/octet-stream";         }     }      private static void InitializeMimeTypeLists()     {         knownTypes = new string[] {         "text/plain",         "text/html",         "text/xml",         "text/richtext",         "text/scriptlet",         "audio/x-aiff",         "audio/basic",         "audio/mid",         "audio/wav",         "image/gif",         "image/jpeg",         "image/pjpeg",         "image/png",         "image/x-png",         "image/tiff",         "image/bmp",         "image/x-xbitmap",         "image/x-jg",         "image/x-emf",         "image/x-wmf",         "video/avi",         "video/mpeg",         "application/octet-stream",         "application/postscript",         "application/base64",         "application/macbinhex40",         "application/pdf",         "application/xml",         "application/atom+xml",         "application/rss+xml",         "application/x-compressed",         "application/x-zip-compressed",         "application/x-gzip-compressed",         "application/java",         "application/x-msdownload"     }.ToList();          mimeTypes = new Dictionary<string, string>();         mimeTypes.Add("3dm", "x-world/x-3dmf");         mimeTypes.Add("3dmf", "x-world/x-3dmf");         mimeTypes.Add("a", "application/octet-stream");         mimeTypes.Add("aab", "application/x-authorware-bin");         mimeTypes.Add("aam", "application/x-authorware-map");         mimeTypes.Add("aas", "application/x-authorware-seg");         mimeTypes.Add("abc", "text/vnd.abc");         mimeTypes.Add("acgi", "text/html");         mimeTypes.Add("afl", "video/animaflex");         mimeTypes.Add("ai", "application/postscript");         mimeTypes.Add("aif", "audio/aiff");         mimeTypes.Add("aifc", "audio/aiff");         mimeTypes.Add("aiff", "audio/aiff");         mimeTypes.Add("aim", "application/x-aim");         mimeTypes.Add("aip", "text/x-audiosoft-intra");         mimeTypes.Add("ani", "application/x-navi-animation");         mimeTypes.Add("aos", "application/x-nokia-9000-communicator-add-on-software");         mimeTypes.Add("aps", "application/mime");         mimeTypes.Add("arc", "application/octet-stream");         mimeTypes.Add("arj", "application/arj");         mimeTypes.Add("art", "image/x-jg");         mimeTypes.Add("asf", "video/x-ms-asf");         mimeTypes.Add("asm", "text/x-asm");         mimeTypes.Add("asp", "text/asp");         mimeTypes.Add("asx", "application/x-mplayer2");         mimeTypes.Add("au", "audio/basic");         mimeTypes.Add("avi", "video/avi");         mimeTypes.Add("avs", "video/avs-video");         mimeTypes.Add("bcpio", "application/x-bcpio");         mimeTypes.Add("bin", "application/octet-stream");         mimeTypes.Add("bm", "image/bmp");         mimeTypes.Add("bmp", "image/bmp");         mimeTypes.Add("boo", "application/book");         mimeTypes.Add("book", "application/book");         mimeTypes.Add("boz", "application/x-bzip2");         mimeTypes.Add("bsh", "application/x-bsh");         mimeTypes.Add("bz", "application/x-bzip");         mimeTypes.Add("bz2", "application/x-bzip2");         mimeTypes.Add("c", "text/plain");         mimeTypes.Add("c++", "text/plain");         mimeTypes.Add("cat", "application/vnd.ms-pki.seccat");         mimeTypes.Add("cc", "text/plain");         mimeTypes.Add("ccad", "application/clariscad");         mimeTypes.Add("cco", "application/x-cocoa");         mimeTypes.Add("cdf", "application/cdf");         mimeTypes.Add("cer", "application/pkix-cert");         mimeTypes.Add("cha", "application/x-chat");         mimeTypes.Add("chat", "application/x-chat");         mimeTypes.Add("class", "application/java");         mimeTypes.Add("com", "application/octet-stream");         mimeTypes.Add("conf", "text/plain");         mimeTypes.Add("cpio", "application/x-cpio");         mimeTypes.Add("cpp", "text/x-c");         mimeTypes.Add("cpt", "application/x-cpt");         mimeTypes.Add("crl", "application/pkcs-crl");         mimeTypes.Add("css", "text/css");         mimeTypes.Add("def", "text/plain");         mimeTypes.Add("der", "application/x-x509-ca-cert");         mimeTypes.Add("dif", "video/x-dv");         mimeTypes.Add("dir", "application/x-director");         mimeTypes.Add("dl", "video/dl");         mimeTypes.Add("doc", "application/msword");         mimeTypes.Add("dot", "application/msword");         mimeTypes.Add("dp", "application/commonground");         mimeTypes.Add("drw", "application/drafting");         mimeTypes.Add("dump", "application/octet-stream");         mimeTypes.Add("dv", "video/x-dv");         mimeTypes.Add("dvi", "application/x-dvi");         mimeTypes.Add("dwf", "drawing/x-dwf (old)");         mimeTypes.Add("dwg", "application/acad");         mimeTypes.Add("dxf", "application/dxf");         mimeTypes.Add("eps", "application/postscript");         mimeTypes.Add("es", "application/x-esrehber");         mimeTypes.Add("etx", "text/x-setext");         mimeTypes.Add("evy", "application/envoy");         mimeTypes.Add("exe", "application/octet-stream");         mimeTypes.Add("f", "text/plain");         mimeTypes.Add("f90", "text/x-fortran");         mimeTypes.Add("fdf", "application/vnd.fdf");         mimeTypes.Add("fif", "image/fif");         mimeTypes.Add("fli", "video/fli");         mimeTypes.Add("flv", "video/x-flv");         mimeTypes.Add("for", "text/x-fortran");         mimeTypes.Add("fpx", "image/vnd.fpx");         mimeTypes.Add("g", "text/plain");         mimeTypes.Add("g3", "image/g3fax");         mimeTypes.Add("gif", "image/gif");         mimeTypes.Add("gl", "video/gl");         mimeTypes.Add("gsd", "audio/x-gsm");         mimeTypes.Add("gtar", "application/x-gtar");         mimeTypes.Add("gz", "application/x-compressed");         mimeTypes.Add("h", "text/plain");         mimeTypes.Add("help", "application/x-helpfile");         mimeTypes.Add("hgl", "application/vnd.hp-hpgl");         mimeTypes.Add("hh", "text/plain");         mimeTypes.Add("hlp", "application/x-winhelp");         mimeTypes.Add("htc", "text/x-component");         mimeTypes.Add("htm", "text/html");         mimeTypes.Add("html", "text/html");         mimeTypes.Add("htmls", "text/html");         mimeTypes.Add("htt", "text/webviewhtml");         mimeTypes.Add("htx", "text/html");         mimeTypes.Add("ice", "x-conference/x-cooltalk");         mimeTypes.Add("ico", "image/x-icon");         mimeTypes.Add("idc", "text/plain");         mimeTypes.Add("ief", "image/ief");         mimeTypes.Add("iefs", "image/ief");         mimeTypes.Add("iges", "application/iges");         mimeTypes.Add("igs", "application/iges");         mimeTypes.Add("ima", "application/x-ima");         mimeTypes.Add("imap", "application/x-httpd-imap");         mimeTypes.Add("inf", "application/inf");         mimeTypes.Add("ins", "application/x-internett-signup");         mimeTypes.Add("ip", "application/x-ip2");         mimeTypes.Add("isu", "video/x-isvideo");         mimeTypes.Add("it", "audio/it");         mimeTypes.Add("iv", "application/x-inventor");         mimeTypes.Add("ivr", "i-world/i-vrml");         mimeTypes.Add("ivy", "application/x-livescreen");         mimeTypes.Add("jam", "audio/x-jam");         mimeTypes.Add("jav", "text/plain");         mimeTypes.Add("java", "text/plain");         mimeTypes.Add("jcm", "application/x-java-commerce");         mimeTypes.Add("jfif", "image/jpeg");         mimeTypes.Add("jfif-tbnl", "image/jpeg");         mimeTypes.Add("jpe", "image/jpeg");         mimeTypes.Add("jpeg", "image/jpeg");         mimeTypes.Add("jpg", "image/jpeg");         mimeTypes.Add("jps", "image/x-jps");         mimeTypes.Add("js", "application/x-javascript");         mimeTypes.Add("jut", "image/jutvision");         mimeTypes.Add("kar", "audio/midi");         mimeTypes.Add("ksh", "application/x-ksh");         mimeTypes.Add("la", "audio/nspaudio");         mimeTypes.Add("lam", "audio/x-liveaudio");         mimeTypes.Add("latex", "application/x-latex");         mimeTypes.Add("lha", "application/lha");         mimeTypes.Add("lhx", "application/octet-stream");         mimeTypes.Add("list", "text/plain");         mimeTypes.Add("lma", "audio/nspaudio");         mimeTypes.Add("log", "text/plain");         mimeTypes.Add("lsp", "application/x-lisp");         mimeTypes.Add("lst", "text/plain");         mimeTypes.Add("lsx", "text/x-la-asf");         mimeTypes.Add("ltx", "application/x-latex");         mimeTypes.Add("lzh", "application/octet-stream");         mimeTypes.Add("lzx", "application/lzx");         mimeTypes.Add("m", "text/plain");         mimeTypes.Add("m1v", "video/mpeg");         mimeTypes.Add("m2a", "audio/mpeg");         mimeTypes.Add("m2v", "video/mpeg");         mimeTypes.Add("m3u", "audio/x-mpequrl");         mimeTypes.Add("man", "application/x-troff-man");         mimeTypes.Add("map", "application/x-navimap");         mimeTypes.Add("mar", "text/plain");         mimeTypes.Add("mbd", "application/mbedlet");         mimeTypes.Add("mc$", "application/x-magic-cap-package-1.0");         mimeTypes.Add("mcd", "application/mcad");         mimeTypes.Add("mcf", "image/vasa");         mimeTypes.Add("mcp", "application/netmc");         mimeTypes.Add("me", "application/x-troff-me");         mimeTypes.Add("mht", "message/rfc822");         mimeTypes.Add("mhtml", "message/rfc822");         mimeTypes.Add("mid", "audio/midi");         mimeTypes.Add("midi", "audio/midi");         mimeTypes.Add("mif", "application/x-frame");         mimeTypes.Add("mime", "message/rfc822");         mimeTypes.Add("mjf", "audio/x-vnd.audioexplosion.mjuicemediafile");         mimeTypes.Add("mjpg", "video/x-motion-jpeg");         mimeTypes.Add("mm", "application/base64");         mimeTypes.Add("mme", "application/base64");         mimeTypes.Add("mod", "audio/mod");         mimeTypes.Add("moov", "video/quicktime");         mimeTypes.Add("mov", "video/quicktime");         mimeTypes.Add("movie", "video/x-sgi-movie");         mimeTypes.Add("mp2", "audio/mpeg");         mimeTypes.Add("mp3", "audio/mpeg3");         mimeTypes.Add("mpa", "audio/mpeg");         mimeTypes.Add("mpc", "application/x-project");         mimeTypes.Add("mpe", "video/mpeg");         mimeTypes.Add("mpeg", "video/mpeg");         mimeTypes.Add("mpg", "video/mpeg");         mimeTypes.Add("mpga", "audio/mpeg");         mimeTypes.Add("mpp", "application/vnd.ms-project");         mimeTypes.Add("mpt", "application/x-project");         mimeTypes.Add("mpv", "application/x-project");         mimeTypes.Add("mpx", "application/x-project");         mimeTypes.Add("mrc", "application/marc");         mimeTypes.Add("ms", "application/x-troff-ms");         mimeTypes.Add("mv", "video/x-sgi-movie");         mimeTypes.Add("my", "audio/make");         mimeTypes.Add("mzz", "application/x-vnd.audioexplosion.mzz");         mimeTypes.Add("nap", "image/naplps");         mimeTypes.Add("naplps", "image/naplps");         mimeTypes.Add("nc", "application/x-netcdf");         mimeTypes.Add("ncm", "application/vnd.nokia.configuration-message");         mimeTypes.Add("nif", "image/x-niff");         mimeTypes.Add("niff", "image/x-niff");         mimeTypes.Add("nix", "application/x-mix-transfer");         mimeTypes.Add("nsc", "application/x-conference");         mimeTypes.Add("nvd", "application/x-navidoc");         mimeTypes.Add("o", "application/octet-stream");         mimeTypes.Add("oda", "application/oda");         mimeTypes.Add("omc", "application/x-omc");         mimeTypes.Add("omcd", "application/x-omcdatamaker");         mimeTypes.Add("omcr", "application/x-omcregerator");         mimeTypes.Add("p", "text/x-pascal");         mimeTypes.Add("p10", "application/pkcs10");         mimeTypes.Add("p12", "application/pkcs-12");         mimeTypes.Add("p7a", "application/x-pkcs7-signature");         mimeTypes.Add("p7c", "application/pkcs7-mime");         mimeTypes.Add("pas", "text/pascal");         mimeTypes.Add("pbm", "image/x-portable-bitmap");         mimeTypes.Add("pcl", "application/vnd.hp-pcl");         mimeTypes.Add("pct", "image/x-pict");         mimeTypes.Add("pcx", "image/x-pcx");         mimeTypes.Add("pdf", "application/pdf");         mimeTypes.Add("pfunk", "audio/make");         mimeTypes.Add("pgm", "image/x-portable-graymap");         mimeTypes.Add("pic", "image/pict");         mimeTypes.Add("pict", "image/pict");         mimeTypes.Add("pkg", "application/x-newton-compatible-pkg");         mimeTypes.Add("pko", "application/vnd.ms-pki.pko");         mimeTypes.Add("pl", "text/plain");         mimeTypes.Add("plx", "application/x-pixclscript");         mimeTypes.Add("pm", "image/x-xpixmap");         mimeTypes.Add("png", "image/png");         mimeTypes.Add("pnm", "application/x-portable-anymap");         mimeTypes.Add("pot", "application/mspowerpoint");         mimeTypes.Add("pov", "model/x-pov");         mimeTypes.Add("ppa", "application/vnd.ms-powerpoint");         mimeTypes.Add("ppm", "image/x-portable-pixmap");         mimeTypes.Add("pps", "application/mspowerpoint");         mimeTypes.Add("ppt", "application/mspowerpoint");         mimeTypes.Add("ppz", "application/mspowerpoint");         mimeTypes.Add("pre", "application/x-freelance");         mimeTypes.Add("prt", "application/pro_eng");         mimeTypes.Add("ps", "application/postscript");         mimeTypes.Add("psd", "application/octet-stream");         mimeTypes.Add("pvu", "paleovu/x-pv");         mimeTypes.Add("pwz", "application/vnd.ms-powerpoint");         mimeTypes.Add("py", "text/x-script.phyton");         mimeTypes.Add("pyc", "applicaiton/x-bytecode.python");         mimeTypes.Add("qcp", "audio/vnd.qcelp");         mimeTypes.Add("qd3", "x-world/x-3dmf");         mimeTypes.Add("qd3d", "x-world/x-3dmf");         mimeTypes.Add("qif", "image/x-quicktime");         mimeTypes.Add("qt", "video/quicktime");         mimeTypes.Add("qtc", "video/x-qtc");         mimeTypes.Add("qti", "image/x-quicktime");         mimeTypes.Add("qtif", "image/x-quicktime");         mimeTypes.Add("ra", "audio/x-pn-realaudio");         mimeTypes.Add("ram", "audio/x-pn-realaudio");         mimeTypes.Add("ras", "application/x-cmu-raster");         mimeTypes.Add("rast", "image/cmu-raster");         mimeTypes.Add("rexx", "text/x-script.rexx");         mimeTypes.Add("rf", "image/vnd.rn-realflash");         mimeTypes.Add("rgb", "image/x-rgb");         mimeTypes.Add("rm", "application/vnd.rn-realmedia");         mimeTypes.Add("rmi", "audio/mid");         mimeTypes.Add("rmm", "audio/x-pn-realaudio");         mimeTypes.Add("rmp", "audio/x-pn-realaudio");         mimeTypes.Add("rng", "application/ringing-tones");         mimeTypes.Add("rnx", "application/vnd.rn-realplayer");         mimeTypes.Add("roff", "application/x-troff");         mimeTypes.Add("rp", "image/vnd.rn-realpix");         mimeTypes.Add("rpm", "audio/x-pn-realaudio-plugin");         mimeTypes.Add("rt", "text/richtext");         mimeTypes.Add("rtf", "text/richtext");         mimeTypes.Add("rtx", "application/rtf");         mimeTypes.Add("rv", "video/vnd.rn-realvideo");         mimeTypes.Add("s", "text/x-asm");         mimeTypes.Add("s3m", "audio/s3m");         mimeTypes.Add("saveme", "application/octet-stream");         mimeTypes.Add("sbk", "application/x-tbook");         mimeTypes.Add("scm", "application/x-lotusscreencam");         mimeTypes.Add("sdml", "text/plain");         mimeTypes.Add("sdp", "application/sdp");         mimeTypes.Add("sdr", "application/sounder");         mimeTypes.Add("sea", "application/sea");         mimeTypes.Add("set", "application/set");         mimeTypes.Add("sgm", "text/sgml");         mimeTypes.Add("sgml", "text/sgml");         mimeTypes.Add("sh", "application/x-bsh");         mimeTypes.Add("shtml", "text/html");         mimeTypes.Add("sid", "audio/x-psid");         mimeTypes.Add("sit", "application/x-sit");         mimeTypes.Add("skd", "application/x-koan");         mimeTypes.Add("skm", "application/x-koan");         mimeTypes.Add("skp", "application/x-koan");         mimeTypes.Add("skt", "application/x-koan");         mimeTypes.Add("sl", "application/x-seelogo");         mimeTypes.Add("smi", "application/smil");         mimeTypes.Add("smil", "application/smil");         mimeTypes.Add("snd", "audio/basic");         mimeTypes.Add("sol", "application/solids");         mimeTypes.Add("spc", "application/x-pkcs7-certificates");         mimeTypes.Add("spl", "application/futuresplash");         mimeTypes.Add("spr", "application/x-sprite");         mimeTypes.Add("sprite", "application/x-sprite");         mimeTypes.Add("src", "application/x-wais-source");         mimeTypes.Add("ssi", "text/x-server-parsed-html");         mimeTypes.Add("ssm", "application/streamingmedia");         mimeTypes.Add("sst", "application/vnd.ms-pki.certstore");         mimeTypes.Add("step", "application/step");         mimeTypes.Add("stl", "application/sla");         mimeTypes.Add("stp", "application/step");         mimeTypes.Add("sv4cpio", "application/x-sv4cpio");         mimeTypes.Add("sv4crc", "application/x-sv4crc");         mimeTypes.Add("svf", "image/vnd.dwg");         mimeTypes.Add("svr", "application/x-world");         mimeTypes.Add("swf", "application/x-shockwave-flash");         mimeTypes.Add("t", "application/x-troff");         mimeTypes.Add("talk", "text/x-speech");         mimeTypes.Add("tar", "application/x-tar");         mimeTypes.Add("tbk", "application/toolbook");         mimeTypes.Add("tcl", "application/x-tcl");         mimeTypes.Add("tcsh", "text/x-script.tcsh");         mimeTypes.Add("tex", "application/x-tex");         mimeTypes.Add("texi", "application/x-texinfo");         mimeTypes.Add("texinfo", "application/x-texinfo");         mimeTypes.Add("text", "text/plain");         mimeTypes.Add("tgz", "application/x-compressed");         mimeTypes.Add("tif", "image/tiff");         mimeTypes.Add("tr", "application/x-troff");         mimeTypes.Add("tsi", "audio/tsp-audio");         mimeTypes.Add("tsp", "audio/tsplayer");         mimeTypes.Add("tsv", "text/tab-separated-values");         mimeTypes.Add("turbot", "image/florian");         mimeTypes.Add("txt", "text/plain");         mimeTypes.Add("uil", "text/x-uil");         mimeTypes.Add("uni", "text/uri-list");         mimeTypes.Add("unis", "text/uri-list");         mimeTypes.Add("unv", "application/i-deas");         mimeTypes.Add("uri", "text/uri-list");         mimeTypes.Add("uris", "text/uri-list");         mimeTypes.Add("ustar", "application/x-ustar");         mimeTypes.Add("uu", "application/octet-stream");         mimeTypes.Add("vcd", "application/x-cdlink");         mimeTypes.Add("vcs", "text/x-vcalendar");         mimeTypes.Add("vda", "application/vda");         mimeTypes.Add("vdo", "video/vdo");         mimeTypes.Add("vew", "application/groupwise");         mimeTypes.Add("viv", "video/vivo");         mimeTypes.Add("vivo", "video/vivo");         mimeTypes.Add("vmd", "application/vocaltec-media-desc");         mimeTypes.Add("vmf", "application/vocaltec-media-file");         mimeTypes.Add("voc", "audio/voc");         mimeTypes.Add("vos", "video/vosaic");         mimeTypes.Add("vox", "audio/voxware");         mimeTypes.Add("vqe", "audio/x-twinvq-plugin");         mimeTypes.Add("vqf", "audio/x-twinvq");         mimeTypes.Add("vql", "audio/x-twinvq-plugin");         mimeTypes.Add("vrml", "application/x-vrml");         mimeTypes.Add("vrt", "x-world/x-vrt");         mimeTypes.Add("vsd", "application/x-visio");         mimeTypes.Add("vst", "application/x-visio");         mimeTypes.Add("vsw", "application/x-visio");         mimeTypes.Add("w60", "application/wordperfect6.0");         mimeTypes.Add("w61", "application/wordperfect6.1");         mimeTypes.Add("w6w", "application/msword");         mimeTypes.Add("wav", "audio/wav");         mimeTypes.Add("wb1", "application/x-qpro");         mimeTypes.Add("wbmp", "image/vnd.wap.wbmp");         mimeTypes.Add("web", "application/vnd.xara");         mimeTypes.Add("wiz", "application/msword");         mimeTypes.Add("wk1", "application/x-123");         mimeTypes.Add("wmf", "windows/metafile");         mimeTypes.Add("wml", "text/vnd.wap.wml");         mimeTypes.Add("wmlc", "application/vnd.wap.wmlc");         mimeTypes.Add("wmls", "text/vnd.wap.wmlscript");         mimeTypes.Add("wmlsc", "application/vnd.wap.wmlscriptc");         mimeTypes.Add("word", "application/msword");         mimeTypes.Add("wp", "application/wordperfect");         mimeTypes.Add("wp5", "application/wordperfect");         mimeTypes.Add("wp6", "application/wordperfect");         mimeTypes.Add("wpd", "application/wordperfect");         mimeTypes.Add("wq1", "application/x-lotus");         mimeTypes.Add("wri", "application/mswrite");         mimeTypes.Add("wrl", "application/x-world");         mimeTypes.Add("wrz", "model/vrml");         mimeTypes.Add("wsc", "text/scriplet");         mimeTypes.Add("wsrc", "application/x-wais-source");         mimeTypes.Add("wtk", "application/x-wintalk");         mimeTypes.Add("xbm", "image/x-xbitmap");         mimeTypes.Add("xdr", "video/x-amt-demorun");         mimeTypes.Add("xgz", "xgl/drawing");         mimeTypes.Add("xif", "image/vnd.xiff");         mimeTypes.Add("xl", "application/excel");         mimeTypes.Add("xla", "application/excel");         mimeTypes.Add("xlb", "application/excel");         mimeTypes.Add("xlc", "application/excel");         mimeTypes.Add("xld", "application/excel");         mimeTypes.Add("xlk", "application/excel");         mimeTypes.Add("xll", "application/excel");         mimeTypes.Add("xlm", "application/excel");         mimeTypes.Add("xls", "application/excel");         mimeTypes.Add("xlsx", "application/excel");         mimeTypes.Add("xlt", "application/excel");         mimeTypes.Add("xlv", "application/excel");         mimeTypes.Add("xlw", "application/excel");         mimeTypes.Add("xm", "audio/xm");         mimeTypes.Add("xml", "text/xml");         mimeTypes.Add("xmz", "xgl/movie");         mimeTypes.Add("xpix", "application/x-vnd.ls-xpix");         mimeTypes.Add("xpm", "image/x-xpixmap");         mimeTypes.Add("x-png", "image/png");         mimeTypes.Add("xsr", "video/x-amt-showrun");         mimeTypes.Add("xwd", "image/x-xwd");         mimeTypes.Add("xyz", "chemical/x-pdb");         mimeTypes.Add("z", "application/x-compress");         mimeTypes.Add("zip", "application/x-compressed");         mimeTypes.Add("zoo", "application/octet-stream");         mimeTypes.Add("zsh", "text/x-script.zsh");     } } 

You have to update this list when a mime type is unknown and it is not sufficient to know that it is application/octet-stream. It might suffice to check if an extension is txt but the mime-type is binary data, because that means that this file is not what it pretends to be.

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Apart from that it's really easier to use a RegularExpressionValidator.

For example (only jpg-images):

<asp:FileUpload ID="FileUpload1" runat="server" /><br /> <br /> <asp:Button ID="Button1" runat="server" OnClick="Button1_Click"   Text="Upload File" />&nbsp;<br /> <br /> <asp:Label ID="Label1" runat="server"></asp:Label> <asp:RegularExpressionValidator   id="RegularExpressionValidator1" runat="server"   ErrorMessage="Only jpeg files are allowed!"   ValidationExpression="^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))     +(.jpg|.JPG|.jpeg|.JPEG)$"   ControlToValidate="FileUpload1"></asp:RegularExpressionValidator> 

In codebehind:

if (FileUpload1.HasFile) {     string fileExt =         System.IO.Path.GetExtension(FileUpload1.FileName);      if (fileExt == ".jpeg" || fileExt == ".jpg")     {         //do what you want with this image     }     else     {         Label1.Text = "Only .jpeg files are allowed!";     } }