js 请求单个文件 并验证扩展名

   function suffix(file_name) {
        var three=file_name.split(".");
        var last=three[three.length-];

        return last;
    }
    $('#btnSearch').click(function(){
        fileElem = document.getElementById("fileElem");
        fileElem.click();
    });

    var  test =false;
    $('#btnImport').click(function () {
        fileElem = document.getElementById("fileElem");
        var name = suffix(fileElem.files[].name);

        if (name.toLowerCase() != "AVI".toLowerCase()) { //判断大小写
            layer.msg("文件类型不对!");
            return false;
        }
        if (fileElem.files.length == ) {
            layer.msg("文件不能为空!");
            return false;
        }
        // 实例化一个表单数据对象
        var uploadFile = new FormData();
        uploadFile.append("A4", fileElem.files[]);
        uploadFile.append("Menuid",@ViewBag.id);
        if(test==false){
            test= true;
            if ("undefined" != typeof (uploadFile) && uploadFile != null && uploadFile != "") {
                $.ajax({
                    url: '/Backstage/',
                    type: 'POST',
                    data: uploadFile,
                    async: false,
                    cache: false,
                    contentType: false, //不设置内容类型
                    processData: false, //不处理数据
                    success: function (data) {
                        layer.msg(data.msg);;
                        setTimeout(function () {
                            test = !test;
                            window.location.href = "/Backstage/";
                        }, );
                        //console.log(data);
                    },
                    error: function () {
                        layer.msg("导入失败!");;
                    }
                })
            } else {
                layer.msg("选择的文件无效！请重新选择");
            }
        }

    });

    var arrEx = ["jpeg","jpg","png","gif"];
   $('#btnImport').click(function () {
        var flag = false; //状态
        fileElem = document.getElementById("fileElem");
        var name = suffix(fileElem.files[0].name);
        //循环比较
        for(var i=0;i<arrEx.length;i++)
        {
            if(name.toLowerCase() == arrEx[i])
            {
                flag = true; //一旦找到合适的，立即退出循环
                break;
            }
        }
        if (!flag) {
            layer.msg("文件类型不对!");
            return false;
        }

C#

                string exName = System.IO.Path.GetExtension(file.FileName); //得到扩展名
                var arrEx =new []{ "jpeg", "jpg", "png", "gif" };
                var flag = false; //状态
                //循环比较
                for (var i = ; i < arrEx.Length; i++)
                {
                    if (exName.ToLower() == arrEx[i])
                    {
                        flag = true; //一旦找到合适的，立即退出循环
                        break;
                    }
                }
                if (!flag) //没有找到
                {
                    return Json(new { msg = "文件类型不对!" });
                }