使用了 el-upload 的插件，但是一直都是报 404 ，在这个过程中，请注意您使用的 request header 中的 content-type 不同的类型需要的 NodeJS 的 request 解析方式不一样。在 Node 服务端接到用户的数据之后，就需要使用 fs 模块把图片解析成可以保存的文件流，存入磁盘中或者调用后台接口保存。调试了一个小时之后成功通过。把代码留下来，方便日后进行查询。

Vue 层
-------------------------------------
<el-upload class="avatar-uploader" accept="image/jpeg" action="agency/uploadPicForAgency" :data="{agencyId:'1',picFlag:'1'}" :multiple="false" :show-file-list="false" :on-success="handleAvatarSuccess" :before-upload="beforeBackgroundAvatarUpload">
  <img v-if="backgroundUrl" :src="backgroundUrl" width="100%" height="100%" class="avatar">
  <i v-else="" class="el-icon-plus avatar-uploader-icon"></i>
</el-upload>

express 层
-----------------------------------
/**
 * @api {post} agency/uploadPicForAgency 代理商图片上传接口，返回图片存入服务器的URL
 * @apiName agency/uploadPicForAgency
 * @apiGroup agency
 * 
 * @apiParam   {FileObject}   imgInfo    图片文件对象
 * @apiParam   {String}       agencyId   代理商ID
 * @apiParam   {String}       picFlag    图片标记（1-登录页logo图，2-登录页背景图，3-内容页logo图）
 * 
 * @apiParamExample  {type} Request-Example:
 * 传统的FORM表单提交
 * Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryzJ4Ys0x4h70Z8heR
 * 
 * ------WebKitFormBoundaryzJ4Ys0x4h70Z8heR
    Content-Disposition: form-data; name="agencyId"

    000192
    ------WebKitFormBoundaryzJ4Ys0x4h70Z8heR
    Content-Disposition: form-data; name="picFlag"

    1
    ------WebKitFormBoundaryzJ4Ys0x4h70Z8heR
    Content-Disposition: form-data; name="file"; filename="Intel-logo.jpg"
    Content-Type: image/jpeg


    ------WebKitFormBoundaryzJ4Ys0x4h70Z8heR--
 * 
 * @apiSuccessExample {Object} Success-Response:
 * {
 *    message: ""
 *    success: true
 *    data: {
 *      imageStr: "/9j/4AAQSkZJRgABAgAAZABkAAD/7AARRHVja3kAAQAEAAAAXA.............."
        uploadPath: "agency/2019/05/54e4c76df1db41bbbca80c9a7ef16c5f.jpg"
      }
 * }
 */
router.post('/uploadPicForAgency', function (req, res, next) {
  const methodName = 'MAgencyPlatformDataAPI.uploadPicForAgency';
  var form = new formidable.IncomingForm();

  form.encoding = 'utf-8'; //设置编辑
  form.keepExtensions = true; //保留后缀
  form.maxFieldsSize = 2 * 1024 * 1024; //设置单文件大小限制
  form.parse(req, function (err, fields, files) {
    const agencyId = fields.agencyId;
    const picFlag = fields.picFlag;
    var imageBuf = fs.readFileSync(files.file.path);
    var imagebase = imageBuf.toString("base64");
    var imgInfo = {
      imgName: files.file.name,
      imgStr: imagebase,
    }
    util.JSONRPC(methodName, [imgInfo,agencyId,picFlag], function (data) {
      res.json({
        success: true,
        data: {
          uploadPath: data,
          imageStr: imagebase
        },
        message: ''
      })
    }, res);
  })
});