js代码
function submitform() {
        $("#form_upload").ajaxSubmit({
            success: showResponse
        });
    }

    function showResponse(responseText) {
        if (responseText != null) {
            alert('上传成功！');
        } else {
            alert('操作失败！');
        }
    }

    $(function () {
        $("#upImg").on("change", function () {
            var file = this.files[0];

            if (this.files && file) {
                var reader = new FileReader();
                reader.onload = function (e) {
                    $('#result').attr('src', e.target.result);
                }
                reader.readAsDataURL(file);
            }
        });
    })

前台代码

                
                    生产(经营)许可证证件照片
                    
                
                
                    
                    
                
            
            
 （这里添加iframe，因为后台返回时会跳转，把form放入iframe里提交就不会跳转页面）

后台代码 
[HttpPost]        
        public ActionResult Upload(HttpPostedFileBase upImg)
        {
            if (upImg == null)
            {
                return Content("文件上传错误，请重新选择文件！");

            }
            string fileName = System.IO.Path.GetFileName(upImg.FileName);
            string filePhysicalPath = Server.MapPath("~/credp_w_picpaths/" + fileName);
            try
            {
                upImg.SaveAs(filePhysicalPath);
                Session["ImgPath"] = filePhysicalPath;
                return Content("上传成功");
            }
            catch
            {
                return Content("上传异常 ！");

            }
        }