GridFs.js

// Importing the required modules
const express = require('express');
const mongoose = require('mongoose');
const read = require('stream-read');
const fs = require('fs');

// initialising the app
const app = express();
const mongoURI = 'XXX'

mongoose.connect(mongoURI, { useNewUrlParser: true }).then(() => console.log('Mongo connected'));

mongoose.connection.once('open', () => {
     //instantiate mongoose-gridfs
     const gridfs = require('mongoose-gridfs')({
        collection:'attachments',
        model:'Attachment',
        mongooseConnection: mongoose.connection
    });
     //obtaining a model
    Attachment = gridfs.model;
    Attachment.write({
        filename:'venom.mp3', 
        contentType:'audio/mpeg'
        }, 
        fs.createReadStream('/home/aditya/Downloads/Eminem - Venom.mp3'), 
        function(error, createdFile){
          console.log('file written')
      });
    // @route: default
    app.get('/', (req, res) => {
        res.send('Hello');
    })
    // @route: streaming music files
    app.get('/music/:id', (req, res) => {
        Attachment.readById(mongoose.Types.ObjectId(req.params.id), (error, content) => {
            if(error) {
                res.json({
                    "error": "Something is not right"
                })
            }
            res.json(content);
        });
    })

})




const port = process.env.PORT || 5000 // for the heroku deployment


app.listen(port, () => console.log('Server is running'))  // creating the server

On the streaming route: I recommend using stream

app.get('/music/:id', (req, res) => {
     res.set('content-type', 'audio/mp3');
     res.set('accept-ranges', 'bytes');
     Attachment.readById(req.params.id).pipe(res);
 })

or 

app.get('/music/:id', (req, res) => {
     res.set('content-type', 'audio/mp3');
     res.set('accept-ranges', 'bytes');
     const stream = Attachment.readById(req.params.id);
     
     stream.on('data', (chunk) => { res.write(chunk); });
     stream.on('error', () => {  res.sendStatus(404); });
     stream.on('end', () => { res.end(); });
 })