RegularExpressionValidator working with FileUpload

<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
ControlToValidate="FileUpload1" ErrorMessage="RegularExpressionValidator"
ValidationExpression="^.*\.(doc|DOC|pdf|PDF)$"></asp:RegularExpressionValidator>
<asp:Button ID="Button5" runat="server" Text="Button" />