Node server using multer to upload files to AWS S3.

server.js

const express = require('express');
const app = express();
const AWS = require('aws-sdk');
const fs = require('fs');
const fileType = require('file-type');
const multiparty = require('multiparty');

// configure the keys for accessing AWS
AWS.config.update({
  accessKeyId: process.env.AWS_ACCESS_KEY_ID,
  secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY
});

// create S3 instance
const s3 = new AWS.S3();

// abstracts function to upload a file returning a promise
const uploadFile = (buffer, name, type) => {
  const params = {
    ACL: 'public-read',
    Body: buffer,
    Bucket: process.env.S3_BUCKET,
    ContentType: type.mime,
    Key: `${name}.${type.ext}`
  };
  return s3.upload(params).promise();
};

// Define POST route
app.post('/test-upload', (request, response) => {
  const form = new multiparty.Form();
    form.parse(request, async (error, fields, files) => {
      if (error) {
        return response.status(500).send(error);
      };
      try {
        const path = files.file[0].path;
        const buffer = fs.readFileSync(path);
        const type = await FileType.fromBuffer(buffer);
        const fileName = `bucketFolder/${Date.now().toString()}`;
        const data = await uploadFile(buffer, fileName, type);
        return response.status(200).send(data);
      } catch (error) {
        return response.status(400).send(error);
      }
    });
});

app.listen(process.env.PORT || 9000);
console.log('Server up and running...');

To find the file type use => const type = await fileType.fromBuffer(buffer);
ref: https://www.npmjs.com/package/file-type