const mongoURI = 'mongodb+srv://caman3874:qwertyuiopaman1234@@amanco-pexfz.mongodb.net/test?retryWrites=true&w=majority';

const promise = mongoose.connect(mongoURI, { useNewUrlParser: true });

const conn = mongoose.connection;
let gfs;

conn.once('open',() => {
  gfs = Grid(conn, mongoose.mongo);
  gfs.collection('uploads');
});

//create storage object
const storage = new GridFsStorage({
  db: promise,
  file: (req, file) => {
    return new Promise((resolve, reject) => {
      crypto.randomBytes(16, (err, buf) => {
        if (err) {
          return reject(err);
        }
        const filename = buf.toString('hex') + path.extname(file.originalname);
        const fileInfo = {
          filename: filename,
          bucketName: 'uploads'
        };
        resolve(fileInfo);
      });
    });
  }
});
const upload = multer({ storage });

希望这行得通！