Uploading audio to mongoDB using GridFS

Question

I would need a second pair of eyes to confirm (or deny) that the following function is working to save an audio blob to a mongo database using GridFS. The fact is that as far as I recall, it has been working, but now it fails to return the file._id. So I wonder what happened or what I am missing.

function saveAudioToGridFS(audioBlob) {
  return new Promise((resolve, reject) => {
    const gridFSBucket = new mongoose.mongo.GridFSBucket(conn.db),
          upldName = getAudioName() // Getting a file name.

    const writeStream = gridFSBucket.openUploadStream(upldName);

    writeStream.on('close', (file) => {
      // For unknown reasons, the value of file is always undefined here.
      if (file !== undefined) {
        console.log("file=(saveAudioToGridFS)",file)
        console.log("file._id=",file._id)
        resolve(file._id); // Return the GridFS file ID
      }
    });

    writeStream.on('finish', (file) => {
      // For unknown reasons, the value of file is always undefined here.
      if (file !== undefined) {
        console.log("file=(finish)",file)
        console.log("file._id=",file._id)
        resolve(file._id); // Return the GridFS file ID
      }
    });

    writeStream.on('error', (error) => {
      reject(error);
    });

  // Convert the Blob into a readable stream using streamifier
  const readableStream = streamifier.createReadStream(audioBlob);
  // audioBlob.pipe(writeStream);
  readableStream.pipe(writeStream);
  });
} /* End of saveAudioToGridFS */

The function above is called in server code like this:

server.post('/upload', async (req, res) => {
  try {
    if (!req.body.audio) {
      return res.status(400).json({ message: 'No audio data uploaded.' });
    }
  
    const audioBuffer = Buffer.from(req.body.audio, 'base64'),
          fileId = await saveAudioToGridFS(audioBuffer);
    .....
  } catch (error) {
    res.status(500).json({
      message: 'An error occurred during upload.',
      error: JSON.stringify(error)
    });
  }
});

When I try it I can check that data is written to the DB, but the file._id is not returned.