Fetch API and multer error while uploading file

I'm trying to use fetch API to upload files to a node.js server (I'm using github's pollyfill if it has something to do with it: https://github.com/github/fetch).

The request is done like this:

const data = new FormData();
data.append('file', file);
return fetch(this.concatToUrl(url), {
  method: 'post',
  headers: Object.assign({}, this.getHeaders(), {'Content-Type': 'multipart/form-data'}),
  body: data,
});

On the server side I have, this declaration of route:

app.post('/media', upload.single('file'), mediaRoutes.postMedia);

And trying to get the file like this:

exports.postMedia = function(req, res) {
  console.log('req.file', req.file, req.files, req.body);
  return res.sendStatus(200);
}

But req.file is not being filled.

Also I'm getting this error from express side:

Error: Multipart: Boundary not found
[2]     at new Multipart (/Users/jmanzano/Development/web/test/node_modules/busboy/lib/types/multipart.js:58:11)
[2]     at Multipart (/Users/jmanzano/Development/web/test/node_modules/busboy/lib/types/multipart.js:26:12)
[2]     at Busboy.parseHeaders (/Users/jmanzano/Development/web/test/node_modules/busboy/lib/main.js:64:22)
[2]     at new Busboy (/Users/jmanzano/Development/web/test/node_modules/busboy/lib/main.js:21:10)
[2]     at multerMiddleware (/Users/jmanzano/Development/web/test/node_modules/multer/lib/make-middleware.js:32:16)
[2]     at Layer.handle [as handle_request] (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/layer.js:95:5)
[2]     at next (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/route.js:131:13)
[2]     at Route.dispatch (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/route.js:112:3)
[2]     at Layer.handle [as handle_request] (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/layer.js:95:5)
[2]     at /Users/jmanzano/Development/web/test/node_modules/express/lib/router/index.js:277:22
[2]     at Function.process_params (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/index.js:330:12)
[2]     at next (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/index.js:271:10)
[2]     at cors (/Users/jmanzano/Development/web/test/node_modules/cors/lib/index.js:178:7)
[2]     at /Users/jmanzano/Development/web/test/node_modules/cors/lib/index.js:228:17
[2]     at originCallback (/Users/jmanzano/Development/web/test/node_modules/cors/lib/index.js:217:15)
[2]     at /Users/jmanzano/Development/web/test/node_modules/cors/lib/index.js:222:13
[2]     at optionsCallback (/Users/jmanzano/Development/web/test/node_modules/cors/lib/index.js:203:9)
[2]     at /Users/jmanzano/Development/web/test/node_modules/cors/lib/index.js:208:7
[2]     at Layer.handle [as handle_request] (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/layer.js:95:5)
[2]     at trim_prefix (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/index.js:312:13)
[2]     at /Users/jmanzano/Development/web/test/node_modules/express/lib/router/index.js:280:7
[2]     at Function.process_params (/Users/jmanzano/Development/web/test/node_modules/express/lib/router/index.js:330:12)

And this is the configuration via middlewares:

app.use(bodyParser.urlencoded({extended: true}));
app.use(bodyParser.json());
app.use(expressValidator());
app.use(logger('dev'));
app.use(cookieParser());
app.use(methodOverride());
app.use(passport.initialize());
app.use(passport.session());
app.set('JWTSuperSecret', jwtConfig.secret);

if (process.env.NODE_ENV !== 'production') {
  app.use(cors());
}

Also, this is correctly working with POSTMAN. So, I think I'm doing something wrong with the request.

Thanks!