执行到request向微信服务器发送请求后，返回来的值是undefined

如下代码第二个then返回undefined，但是用开发者调试工具上传临时素材却可以成功上传并获得mediaId，说明accesstoken应该没问题，相关代码如下：

.then(function(data){
                var url = api.upload + '&access_token=' + data + '&type=' + type;
                console.log(url,form)
                request({
                    method:'POST',
                    url:url,
                    json:true,
                    formData:form,
                })
            })
            .then(function(res){
                console.log('返回的：',res);
            })

在第一个then中打印出的url与调试工具中的url相同，说明url正确。问题可能出在form上，我将form打印出来如下：

{ media:
   ReadStream {
     _readableState:
      ReadableState {
        objectMode: false,
        highWaterMark: 65536,
        buffer: [Object],
        length: 65083,
        pipes: null,
        pipesCount: 0,
        flowing: null,
        ended: true,
        endEmitted: false,
        reading: false,
        sync: false,
        needReadable: false,
        emittedReadable: true,
        readableListening: false,
        resumeScheduled: false,
        defaultEncoding: 'utf8',
        ranOut: false,
        awaitDrain: 0,
        readingMore: false,
        decoder: null,
        encoding: null },
     readable: true,
     domain: null,
     _events: { end: [Function] },
     _eventsCount: 1,
     _maxListeners: undefined,
     path: 'C:\\Users\\Administrator\\node\\task\\day05\\wechat\\金刚狼.jpg',
     fd: 3,
     flags: 'r',
     mode: 438,
     start: undefined,
     end: undefined,
     autoClose: true,
     pos: undefined } 
 }