title: Flutter Upload image with MultipartRequest inside specific body
body: <p>I need to upload a list of attachments inside an array with this structure:</p>
  <pre><code>[
  'description' =&gt; ['sometimes', 'string'],
  'attachments' =&gt; ['sometimes', 'array'],
  'attachments.*.file' =&gt; ['required', 'file'],
  ]
  </code></pre>
  <p>I'm using package http, and to upload files only allow me to put inside 'files', i'm using this code and it works to upload a file inside files, but I need that structure.</p>
  <pre><code> if (json != null) {
        request.fields.addAll(json);
      }
  
      for (var item in filesPath) {
        request.files.add(await http.MultipartFile.fromPath(
          'file',
          item,
        ));
      } 
  </code></pre>
  <p>There is any way to achieve this with this plugin ?</p>