FromData上传ajax，请求发出了，服务器却没反应

qzutil.getPDF(-8,0.9,function(doc){
            var formData=new FormData();
            var positions=qzutil.getStampPositons(-8,0.9);
            formData.append("positions",positions);
            formData.append("doc",doc);
             $.ajax({
                 url: "/ybc/qianzhang/doQianzhang.do",
                 type: "POST",
                 data: formData,//改成new DataForm()服务器才有反应
                 contentType: false,
                 processData: false,
                 success: function (data) {
                     if (data["data"] == "true") {
                         alert("上传成功！");
                     }
                     if (data["data"]=="error") {
                         alert("上传失败");
                     }
                     
                 },
                 error: function () {
                     alert("上传失败！");
                 }
         });
        });