如何在ASP.NET AJAX Control Toolkit中的AsyncFileUpload的文件浏览菜单中限制文件类型

<ajaxToolkit:AsyncFileUpload ID="AsyncFileUpload1" runat="server" AllowedFileTypes="jpg,jpeg,png,gif" />

<ajaxToolkit:AsyncFileUpload ID="AsyncFileUpload1" runat="server" AllowedFileTypes="jpg,jpeg,png,gif" OnClientUploadStarting="uploadStarting" /><script type="text/javascript">
    function uploadStarting(sender, args) {
        var fileExtension = args.get_fileName().split('.').pop();
        if (fileExtension != "jpg" && fileExtension != "jpeg" && fileExtension != "png" && fileExtension != "gif") {
            alert("Invalid file type. Please upload a .jpg, .jpeg, .png, or .gif file.");
            args.set_cancel(true);
        }
    }
</script>