在node-formidable上的文档看到可以这样来限制上传大小

form.maxFieldsSize = 2 * 1024 * 1024;

但是我代码这样子写，依旧是没有限制到上传的大小，现在不管多大都能上传。求指导。谢谢！

router.post('/up', function (req, res) {
  var form = new formidable.IncomingForm();
  form.encoding = 'utf-8';
  form.uploadDir = 'public/images/';
  form.keepExtensions = true;
  form.maxFieldsSize = 2 * 1024;
  console.log(form);
  form.parse(req, function (err, fields, files) {
    console.log(fields);
    console.log(files);
    if (err) {
      return console.log(err);
    }
    var extName = '';
    switch (files.img.type) {
    case 'image/pjpeg':
      extName = 'jpg';
      break;
    case 'image/jpeg':
      extName = 'jpg';
      break;
    case 'image/png':
      extName = 'png';
      break;
    case 'image/x-png':
      extName = 'png';
      break;
    }
    if (extName.length == 0) {
     // return console.log("no img");
    }
    var imgName = uuid.v1() + extName;
    var newPath = form.uploadDir + imgName;
    console.log(newPath);
    fs.renameSync(files.img.path, newPath);
    console.log("ok");
    res.end(util.inspect({fields: fields, files: files}));
  });

  form.on('fileBegin', function (name, value) {
    console.log('....');
    console.log(name);
    console.log('....');
    console.log(value);
  });

  form.on('end', function () {
    console.log("en");
  });

  form.on('error', function (err) {
    console.log(err);
    console.log("error");
  });

  form.on('file', function (name, file) {
    console.log("```````");
    console.log(name);
    console.log(file);
  });

  form.on('progress', function (bytesReceived, bytesExpected) {
    var percent = Math.floor(bytesReceived / bytesExpected * 100);
    console.log(percent);
  });


});