heroku上的节点文件系统访问错误

Question

我有一个简单的代码，使用express fs

将选择的图像保存到文件系统

exports.upload = function(req, res) {

  var photoInfo = req.body,
      file      = req.files.file,
      fileType  = file.type.slice(6);

  //attaching the fileType to photoInfo because fileType is part of req.files, not part of req.body
  photoInfo.fileType = fileType;

  var newPhoto = new Photo(photoInfo);

  Photo.create(newPhoto, function(err, insertedPhotoInfo){
    if (err) throw err;

    var photoId = insertedPhotoInfo._id;

    //create directory if it does not exist already
    fs.exists(__directory, function(exists) {
      if (!exists) {
        fs.mkdir(__directory, function(err, data){
          if (err) throw err;
        });
      }
    });

    //create the file with the unique id created by mongo as its name
    // http://www.hacksparrow.com/handle-file-uploads-in-express-node-js.html 
    // get the temporary location of the file
    var tmpPath = file.path;
    // set where the file should actually exists - in this case it is in the "images" directory
    // var targetPath = './uploads/' + req.files.file.name;
    // move the file from the temporary location to the intended location
    fs.rename(tmpPath, __directory + '/' + photoId + '.' + fileType, function(err){
      if (err) throw err;
      // var reader = new FileReader();
      // delete the temporary file, so that the explicitly set temporary upload dir does not get filled with unwanted files
      fs.unlink(tmpPath, function() {
        if (err) throw err;
        res.send(insertedPhotoInfo);
      });
    });
  });
};

这在localhost上完美运行，但是一旦我部署到heroku，我就会收到ENOENT，打开错误。它似乎失去了对fs的访问权限。我认为这是因为heroku只有短暂的文件系统。

为此会有任何解决方法吗？或者我需要设置一个单独的vm /静态文件服务器？