When the lib receives files and fields, it starts to receive (upload) files in any case.
For example, I check form fields for emptiness, and reject the request. But it still accepts the files and stores them, and I can't prevent it.

form.on("field", function (name, value) {
  // Reject if a form field value is empty
  if (_.isEmpty(value)) {
    return res.sendStatus(400);
    // Want to cancel processing of the form here, but the lib continues
  }
});
form.on("file", function (name, value) {
  // And accepts the file, stores it, and reaches here
});

So in this example, if the received form consists of an empty field, and a file, its not possible to stop the file from being accepted (correct me if I've mistaken).