code: function buildData(context) {
  
      if (/^Head|Get|Delete$/.test( context.method ))  return '';
  
      const request = context.request;
  
      if (request.type !== 'multipart/form-data')  return request.body;
  
      const form = new FormData();
  
      for (let key in request.fields)  form.append(key, request.fields[ key ]);
  
      for (let key in request.files)
          request.files[ key ].forEach(file =>
              form.append(key,  createReadStream( file.path ),  file.name)
          );
  
      return form;
  }
docs: @external {Context} https://github.com/koajs/koa/blob/master/docs/api/context.md
   
  @param {Context} context
  
  @return {FormData|Object|string}