How to restrict file type in FileUpload control

You cannot strictly restrict the file type, but if the browser supports it you can cause it to initially show just a certain type of file:

<form method="post" action="blahblah.blah">
  <input type="file" name="image" id="image" accept="image/png, image/jpeg" />
</form>

I found no direct solution for this problem.

This is my workaround using the RegularExpressionValidator:

<asp:FileUpload ID="fuImportImage" runat="server" />
<asp:RegularExpressionValidator ID="regexValidator" runat="server"
     ControlToValidate="fuImportImage"
     ErrorMessage="Only JPEG images are allowed" 
     ValidationExpression="(.*\.([Jj][Pp][Gg])|.*\.([Jj][Pp][Ee][Gg])$)">
</asp:RegularExpressionValidator>

In 2015, web browsers support the input accept attribute, so you can do this:

<asp:FileUpload ID="fileUploader" runat="server" accept=".png,.jpg,.jpeg,.gif" />

Keep in mind Visual Studio may show you a message about this as an invalid attribute of the FileUpload ASP tool, however.