RegularExpressionValidator
<asp:RegularExpressionValidator runat="server" ErrorMessage="Only picture files are allowed!" ValidationExpression="^(([a-zA-Z]:)|(
\\{2}\w+)\$?)(\\(\w[\w].*))+(.gif|.GIF|.jpg|.JPG|.jpeg|.JPEG)$" ControlToValidate="fileUpload" Display="None"></asp:RegularExpressionValidator>