文件上传multer能接收到的文件对象为什么formidable会解析成fields对象呢

Question

/*var multer  = require('multer');

var upload = multer({ storage: storage });

router.post('/', upload.single('file'), function(req, res, next) {

        console.log('文件',req.body.file);

}*/

而使用formidable来解析

var formidable = require('formidable');

var form = new formidable.IncomingForm();

    //缓存路径

    form.uploadDir = "./temp";

    form.keepExtensions = false;

    //回调函数的files参数将会是一个file数组

    form.multiples = true;

    form.parse(req,function(err, fields, files){

        //console.log('req',req);

        //if(err) return next(err);

        console.log(err,JSON.stringify(fields),files);

    }

打印出的数据为    null '{"file":"[object HTMLInputElement]"}' {}

为什么会解析成files对象呢，求教大佬。

html:

<input type="file" />

<a href="javascript:void(0)">提交</a>

var Input = document.querySelector("input");

var A = document.querySelector('a');

A.onclick = function(){

    // 创建一个FormData对象,用来组装一组用 XMLHttpRequest发送请求的键/值对

    var fd = new FormData();

    fd.enctype = "multipart/form-data";

    fd.append('file', Input);

    // Ajax

    var request = new XMLHttpRequest();

    request.open("POST", "/index",true);

    //request.setRequestHeader("Content-type","application/form-data");

    request.send(fd);

    request.onreadystatechange = function(){

        if(request.readyState === 4 & request.status === 200){

            console.log("上传成功");

            var response = JSON.parse(request.responseText);

            console.log(response);

        }

    }

}