RegularExpressionValidator doesn't display the message when a wrong type of file is selected

Hi,
I have this code for the asp:RegularExpressionValidator but the message is not displayed.
<asp:RegularExpressionValidator id="FileUpLoadValidator" runat="server" ErrorMessage="Upload csv files only." ValidationExpression="^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))(.csv)$" ControlToValidate="FileUpload1" ForeColor="Red"></asp:RegularExpressionValidator>
Can anybody help me to find out why? Thanks