append(key: string, value: string | File): void

import http from 'pts/http';

const data = open('./sample/tmp.js');

export default function () {
  const formData = new http.FormData();
  
  // Add data with value as string.
  formData.append('text', 'text');
  // Add data with value as File.
  formData.append('file', http.file(data, 'tmp.js'));
  
  const resp = http.post('http://httpbin.org/post', formData.body(), {
    headers: {'Content-Type': formData.contentType()}
  });
  console.log('formData: ', resp.body);
};