How do I Validate the File Type of a File Upload?

Question

I am using <input type="file" id="fileUpload" runat="server"> to upload a file in an ASP.NET application. I would like to limit the file type of the upload (example: limit to .xls or .xlsx file extensions).

Both JavaScript or server-side validation are OK (as long as the server side validation would take place before the files are being uploaded - there could be some very large files uploaded, so any validation needs to take place before the actual files are uploaded).

Answer 1

Seems like you are going to have limited options since you want the check to occur before the upload. I think the best you are going to get is to use javascript to validate the extension of the file. You could build a hash of valid extensions and then look to see if the extension of the file being uploaded existed in the hash.

HTML:

<input type="file" name="FILENAME"  size="20" onchange="check_extension(this.value,"upload");"/> <input type="submit" id="upload" name="upload" value="Attach" disabled="disabled" /> 

Javascript:

var hash = {   'xls'  : 1,   'xlsx' : 1, };  function check_extension(filename,submitId) {       var re = /\..+$/;       var ext = filename.match(re);       var submitEl = document.getElementById(submitId);       if (hash[ext]) {         submitEl.disabled = false;         return true;       } else {         alert("Invalid filename, please select another file");         submitEl.disabled = true;          return false;       } }