Validate file upload field in ASP.NET using Regular Expression Validator

<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="FileUpload1"
  Display="None" ErrorMessage="Please select a file" SetFocusOnError="True" />
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ErrorMessage="Only pdf files allowed." ValidationExpression="^.+\.(([pP][dD][fF]))$" ControlToValidate="FileUpload1" />