<asp:RegularExpressionValidator 
id="FileUpLoadValidator" runat="server" 
ErrorMessage="Upload Jpegs and Gifs only." 
ValidationExpression="^(([a-zA-Z]:)|(//{2}/w+)/$?)(//(/w[/w].*))(.jpg|.JPG|.gif|.GIF)$" 
ControlToValidate="FileUpload1"> 
</asp:RegularExpressionValidator>