RegularExpressionValidator not working with FileUpload

I have a RegularExpresssionValidator, i thought, checking my FileUpload control. I know that it was working with some initial testing, but noticed that my client was able to upload a word document. Below is the controls within my ASPX page, in my code behind i only have code to upload the file, no checking there. But seems i need to add more validation to make sure that only PDF files are allowed.  <td> <asp:FileUpload ID="pdfupload" runat="server" Width="500px" /> <asp:Button ID="pdfup" runat="server" CausesValidation="False" onclick="pdfup_Click" Text="Upload PDF" /> <br /> <asp:RegularExpressionValidator ID="regPDF" ru ...