Typical FileUpload RegularExpressionEvaluator Triggers Error, how to fix? - asp.net

Question

On my page I have an standard asp:FileUpload control. Right below it is a RegularExpressionValidator that is supposed to only allow pdf and Word documents to be uploaded. It's the same regex seen all around the web. For some reason, it worked fine in aother project, but not in my current one. When loading the page in Firefox, I immediately get the validator to show me the error message right after selecting the file. This tells me it's a client side issue as the page has yet to be submitted to the server.
The tag:
<asp:RegularExpressionValidator id="Resume_Validator" runat="server"
ControlToValidate="Resume"
ValidationGroup="applicationForm"
ErrorMessage="Upload PDF or Word files only<br />"
Display="Dynamic"
CssClass="validationMsg"
ValidationExpression="^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))(.pdf|.PDF|.doc|.DOC|.docx|.DOCX)$"
/>

Answer 1

Why not just use jQuery file validation on the client-side?
//Check file extension
var ext = file.split('.').pop().toLowerCase(); //Check file extension if valid or expected
if ($.inArray(ext, ['txt']) == -1) {
$(".errorDiv").html("Select valid text file (txt).");
e.preventDefault(); //Prevent submission of form
}
else {
//Do your logic here, file upload, stream. etc.. if file was successfully validated
}