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!" });  
            }