springmvc下使用formdata异步ajax上传图片

最近学习springmvc框架，其中一项需求是产品修改页异步上传修改产品图片，使用的是FormData+Jquery，亲测有效，除了ajax返回结果乱码没处理，下面贴代码

SpringMVC配置代码，需要配置文件上传处理器，我的是springmvc-servlet.xml：

id="multipartResolver"
      class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
    name="maxUploadSize">
        5242880
    

JS及表单部分,功能单一，没有使用其它的js，除了jquery,标红的部分是我遇到的问题（在水狐提示uncaught exception:out of memory,请求失败）：

function asyncUploadFile() {
  var fd = new FormData(document.getElementById("form1"));
  $.ajax({
    url: "${pageContext.request.contextPath}/img/upload",
    type: "POST",
    data: fd,
    dataType:"json",
    processData: false,  // 告诉jQuery不要去处理发送的数据
    contentType: false ,  // 告诉jQuery不要去设置Content-Type请求头
    success:function(data){
      alert(data.msg);
    },
    error:function(){
      alert("error");
    }
  });
}

id= "form1" method= "post" enctype= "multipart/form-data" >

  type="file" name="pic_up"/>
  type="button" οnclick="asyncUploadFile()">异步上传图片测试

Controller部分：

@RequestMapping("/upload")
public void upload(HttpServletRequest request, HttpServletResponse response, @RequestParam MultipartFile pic_up) throws Exception {
    String orginalName = pic_up.getOriginalFilename();
    if (pic_up != null && orginalName != null && orginalName.length() > 0) {
        String imgDir = request.getServletContext().getRealPath("attached/image");
        String suffix = orginalName.split("\\.")[1];
        String imgName = new SimpleDateFormat("yyyyMMddHHmmss").format(new Date()) + "_" + new Random().nextInt(1000) + "." + suffix;
        String imgurl = "/attached/image/" + imgName;
        File file = new File(imgDir, imgName);
        pic_up.transferTo(file);
        JSONObject obj  = new JSONObject();
        obj.put("code",1);
        obj.put("msg","上传成功");
        response.getWriter().write(JSON.toJSONString(obj));
    }
}

控制台DEBUG，显示上传成功:

DEBUG [http-bio-80-exec-6] - Multipart file 'pic_up' with original filename [1(1).jpg], stored at [C:\Users\Administrator\.IntelliJIdea2016.1\system\tomcat\tomcat_jiumu_gps\work\Catalina\localhost\_\upload_6b5fac79_154bcd2cb8e__7fd4_00000000.tmp]: moved to [D:\_workspace\jiumu_gps\out\artifacts\web_war_exploded\attached\image\20160517114706_110.jpg]