fileupload的问题只允许图像

                <asp:RegularExpressionValidator 
 id="RegularExpressionValidator1" runat="server" 
 ErrorMessage="Only images are allowed!" 
 ValidationExpression="^(([a-zA-Z]:)|(\{2}\w+)$?)(\(\w[\w].*))
    +(.jpg|.JPG|.gif|.GIF|.png|.PNG|.jpeg|.JPEG)$" 
 ControlToValidate="fileUpEx"></asp:RegularExpressionValidator>

                <asp:FileUpload ID="fileUpEx" runat="server" />
                <asp:RegularExpressionValidator