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