layui upload 后台获取不到值

后台获取不到值方法一：

<script>
    layui.use('upload', function () {
        var upload = layui.upload;

        //执行实例
        var uploadInst = upload.render({
            elem: '#test1' //绑定元素
            , url: '/Home/UploadFiles' //上传接口
             , field: "fileNames"     //添加这个属性与后台名称保存一致
            , accept: 'file' //允许上传的文件类型
            , size: 500000 //最大允许上传的文件大小
            , before: function (obj) { //obj参数包含的信息，跟 choose回调完全一致，可参见上文。
                layer.load(); //上传loading
            }
            , done: function (res) {
                //上传完毕回调
                alert(res.tc);
            }
            , error: function () {
                //请求异常回调
            }
        });
    });
</script>

 public ActionResult UploadFiles(HttpPostedFileBase fileNames)
        {
            string path = "";
            //小于20M
            if (fileNames.ContentLength > 0 && fileNames.ContentLength <= 120971520)
            {

                var fileName = Path.GetFileName(fileNames.FileName);

                string q_FN = fileName.Substring(0, fileName.LastIndexOf("."));
                string h_FN = fileName.Substring(fileName.LastIndexOf("."));

                string NewFileName = q_FN + DateTime.Now.ToString("yyyyMMddHHmmss") + h_FN;

                path = Path.Combine(Server.MapPath("/Uploadfile/"), NewFileName);
                fileNames.SaveAs(path);


                path = "/Uploadfile/" + NewFileName;
                var relt = new { tc = path };
                return Content(JsonConvert.SerializeObject(relt));
            }
            else
            {
                var relt = new { tc = "上传文件要小于20M" };
                return Content(JsonConvert.SerializeObject(relt));
            }
        }