FormData sending in a post request via Http.post

I have a structure like this:

let post = new FormData();
post.append('content', document.querySelector('.textfield').innerHTML);
post.append('creator', creator);
post.append('post_date', Date.now());

and I try to make request with this form data above:

this.postsService.addPost(post).subscribe(data => {});

postsService:

let headers = new Headers();
headers.append('Content-Type', 'multipart/form-data');
return this.http.post('http://localhost:3000/posts/add', post, 
{headers: headers}).map(res => res.json());

and in backend, in node.js I get req.body as undefined:

router.post('/add', (req, res) => {
  console.log(req.body); //undefined
}

Why backend does not approve my FormData object?