title: FormData append nested object
body: <p>Is it possible to append nested object to <code>FormData</code>?</p>
  
  <pre><code>let formData = new FormData();
  let data = {
      title: 'title',
      text: 'text',
      preview: {p_title:'p title', p_text: 'p text'}
  };
  
  $.each(data, function(key, value) {
      formData.append(key, value);
  });
  </code></pre>
  
  <p><strong>Server console</strong> - console.log(req.body)</p>
  
  <pre><code>{
      title: 'title',
      text: 'text',
      preview: '[object Object]'
  }
  </code></pre>
  
  <p>How can I get the exact value of <code>preview: {p_title:'p title', p_text: 'p text'}</code>?</p>