index.js

var express = require('express');
var router = express.Router();
var formidable = require('formidable');
var Uploader = require('../helpers/s3-uploader');

router.post('/enviar', function (req, res, next) {
  var form = new formidable.IncomingForm();
  form.parse(req, function(err, fields, files) {
    if (err) {
      return next(err);
    }
    var uploader = new Uploader();
    uploader.upload(files.foto, 600)
      .then(function (urlDaFoto) {
        var meuObjeto = new MeuObjeto({
          urlDaFoto: urlDaFoto
        });
        meuObjeto.save()
          .then(function () {
            res.redirect('/');
          })
          .catch(function (err) {
            next(err);
          });
      })
      .catch(function (err) {
        next(err);
      });
  });
});

s3-uploader.js

var fs = require('fs');
var path = require('path');
var os = require('os');
var AWS = require('aws-sdk');
var im = require('gm').subClass({ imageMagick: true });
var uuid = require('node-uuid');
var Promise = require('bluebird');
var config = require('./config');

function S3Uploader() {
  this.s3 = new AWS.S3({
    accessKeyId: config.aws.accessKeyId,
    secretAccessKey: config.aws.secretAccessKey
  });
  this.tmpdir = os.tmpdir();
}

S3Uploader.prototype.upload = function (file, width, height) {
  var self = this;
  return new Promise(function (resolve, reject) {
    self._resize(file.path, width, height)
      .then(function (fileName) {
        var params = {
          Bucket: config.uploadsBucket,
          Key: path.join('uploads', path.basename(fileName)),
          Body: fs.createReadStream(fileName),
          ContentType: file.type
        };
        self.s3.upload(params, function (err, data) {
          if (err) {
            reject(err);
          } else {
            resolve(data.Location);
          }
        });
      })
      .catch(function (err) {
        return Promise.reject(err);
      });
  });
};

S3Uploader.prototype._resize = function (file, width, height) {
  if (!height) {
    height = width;
  }
  var tempName = this._tempName(file);
  return new Promise(function (resolve, reject) {
    im(file).resize(width, height).quality(80).write(tempName, function (err) {
      if (err) {
        reject(err);
      } else {
        resolve(tempName);
      }
    });
  });
};

S3Uploader.prototype._tempName = function (file) {
  return path.join(this.tmpdir, uuid.v4() + path.extname(file));
};

module.exports = S3Uploader;