Error on Heroku while uploading images to mongoDB using Multer

I have developed an app where the user can upload images using multer and can retrieve/view them on the home screen. I'm using MongoDB as my database, locally it works perfectly. Even when using MongoDB CL, It's running well locally. But when I deploy it to Heroku with the database connection link, I'm getting the following error.

2020-07-06T16:21:38.008989+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=quiet-anchorage-51950.herokuapp.com request_id=146ff967-98bb-45e5-a234-4dbf9535abbe fwd="124.123.143.110" dyno= connect= service= status=503 bytes= protocol=https

Here is my app.js:

const express = require('express');
const app = express();
const bodyParser = require('body-parser');
const mongoose = require('mongoose');
const fs = require('fs');
var path = require('path');
var multer  = require('multer');

var storage = multer.diskStorage({
    destination: (req, file, cb) => {
        cb(null, 'uploads')
    },
    filename: (req, file, cb) => {
        cb(null, file.fieldname + '-' + Date.now())
    }
});
var upload = multer({ storage: storage })

app.use(express.static("public"));
app.use(bodyParser.urlencoded({ extended: false }));
app.set("view engine", "ejs");

mongoose.connect("mongodb+srv://<myname>:<removedpassword>@webdatabase.rbrhg.mongodb.net/<dbname>?retryWrites=true&w=majority", {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

The image Schema I have used:

const imgSchema = new mongoose.Schema({
    img: {
        data: Buffer,
        contentType: String
    }
});

The post request I made to upload the images:

app.post('/imgupload', upload.single('image'), function (req, res, next) {
    var image = new Image({
        img: {
            data: fs.readFileSync(path.join(__dirname + '/uploads/' + req.file.filename)),
            contentType: 'image/png'
        }
    });
    image.save();
    res.render("app",{Name: req.body.username});
});

Here is my package.json file:

{
  "name": "igwebsite",
  "version": "1.0.0",
  "description": "",
  "main": "app.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "node app.js"
  },
  "author": "",
  "license": "ISC",
  "engines": {
    "node": "12.16.3"
  },
  "dependencies": {
    "body-parser": "^1.19.0",
    "dotenv": "^8.2.0",
    "ejs": "^3.1.3",
    "express": "^4.17.1",
    "mongodb": "^3.5.9",
    "mongoose": "^5.9.21",
    "multer": "^1.4.2"
  }
}

The Thing is I want my locally-perfect web app to work online, Can someone please help?