title: how to validate only files are pdf?
body: <p>I need to create a regular expression in a FileUpload,
  that only allow pdf files </p>
  
  <pre><code>&lt;asp:RegularExpressionValidator runat="server" ID="valUpTest" ControlToValidate="FileUpload1"
                             ErrorMessage="Files Only (.pdf)" ValidationExpression="?" /&gt;
  </code></pre>