js 验证上传文件格式

<script type="text/javascript">

        function yanzheng2()
        {
            var a=document.getElementById("TextBox1");
            if(a.value=="")
            {
                alert('请输入购买个数！');
                a.focus();
                return false;
            }
            var b=document.getElementById("FileUpload1");
            if(b.value=="")
            {
                alert('请选择上传图片！');
                b.focus();
                return false;
            }
            else
            {
                var FileName=new String(b.value);//文件名
                var extension=new String (FileName.substring(FileName.lastIndexOf(".")+1,FileName.length));//文件扩展名
                if(extension=="jpg"||extension=="gif"||extension=="jpeg"||extension=="bmp")
                {}
                else
                {
                    alert('产品图片格式不支持！');
                    return false;
                }
            }
            var c=document.getElementById("xieyi");
            if(c.checked==false)
            {
                alert('请查看并同意购买协议！');
                c.focus();
                return false;
            }
            return true;
        }

</script>

<asp:ImageButton ID="Button1" ImageUrl="~/images/fuwu/tongyii.gif" runat="server" onmousedown="return yanzheng2()" width="90" height="32"
            onclick="Button1_Click" />