In my project , I use the XMLHttpRequest to send multipart/form-data. I think it will fit you to.
and the uploader code
let xhr = new XMLHttpRequest();
xhr.open('POST', 'http://www.example.com/rest/api', true);
xhr.withCredentials = true;
xhr.send(formData);
Here is example : https://github.com/wangzilong/angular2-multipartForm