vue2上传文件，用axios发送FormData数据,参数为空的问题，设置withCredentials: true报跨域错误

upload(event) {

        var that=this;

        that.file=event.target.files;

        let  formData=newFormData();  

        formData.append("uid", that.loginUserId); 

        formData.append("key", that.loginUserKey); 

        formData.append("key_type", "1");

        formData.append("id", that.accountID);

        formData.append("price", that.money*100);

        formData.append("money", row.price_title);

        formData.append("QR_url",that.file);   

        constinstance=axios.create({

        withCredentials:true

                 })        

        instance.post('/pay/keep_qr.html',formData).then(res\=>{

            if(res.code==200){
                  alert("上传成功");
                     }

               });

    },

虽然显示报错，但是实际操作调接口是成功的，图片已传到服务器。