<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>