Checking file type from base64?

Question

I have a WCF REST Service with the following OperationContract that saves files on the disk:

[OperationContract]
[WebInvoke(UriTemplate = "FileSave", Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.WrappedRequest)]
ResponseHandler FileSave(string fileName, string fileContent);

Files are sent through javascript - using HTML File API => binary data => base-64 encoded ASCII string (=fileContent is recieved in the operation contract)

I want to check the file type before saving the file on the disk. I am aware of Checking MIME Type from a base64 string on the Code Review Stack Exchange but I am not sure if it is the best way to go. Also, I have tested uploading several .txt files and each one has different first 5 chars.

I am looking for a code snippet that would include checking for several common file types.

Answer 1

Check this link here:

https://web.archive.org/web/20170331115315/http://codeanalyse.com/2016/10/02/extracting-file-extension-base64-string/

This "would include checking for several common file types"

/// <summary>
/// To demonstrate extraction of file extension from base64 string.
/// </summary>
/// <param name="base64String">base64 string.</param>
/// <returns>Henceforth file extension from string.</returns>
public static string GetFileExtension(string base64String)
{
var data = base64String.Substring(0, 5);

switch (data.ToUpper())
 {
     case "IVBOR":
        return "png";
     case "/9J/4":
         return "jpg";
     case "AAAAF":
         return "mp4";
     case "JVBER":
         return "pdf";
     case "AAABA":
         return "ico";
     case "UMFYI":
         return "rar";
     case "E1XYD":
         return "rtf";
     case "U1PKC":
        return "txt";
     case "MQOWM":
     case "77U/M":
        return "srt";
     default:
        return string.Empty;
 }
}

Note: If you are using a web browser upload process, the string may have data:image/png;base64, at the start. This should be stripped out first. This part of the string cannot be trusted for web uploads because libraries will add this based on the extension, not on the actual file type. I.e. an excel file named accounts.xlsx.pdf will be marked as a PDF, not xlsx. Looking at the content as shown above is a more trustable way to evaluate the file.