docs: def send_letter(self, file_list, destination='DE', duplex=True, color=False, user_transaction=None,
                      gogreen=False,test_environment=False):
          """
          Send pdf-File
          :param file_list: list of files
          :param destination: country code of the destination country
          :param duplex: send letter in duplex
          :param color: send letter with color or black/white
          :param user_transaction: custom transaction id
          :param test_environment: Enable Test-Mode
          :return: Letter-ID if letter was sended successfully, else None
          """
  
          idx = 0
          last = len(file_list) - 1
          multiple_files = True if len(file_list) > 1 else False
          letter_id = None
          for file in file_list:
              files = {'file': file}
              if idx == 0:
                  data = {
                      'settings[destination]': destination,
                      'settings[simplex]': 'NONE' if duplex else 'ALL',
                      'settings[color]': 'ALL' if color else 'NONE',
                      'settings[user_transaction]': user_transaction,
                      'settings[test_environment]': test_environment,
                      'settings[gogreen]': True if gogreen else False,
                      'incomplete': True if multiple_files else False
                  }
                  send_req = self._make_post_request('letters', data=data, files=files)
  
                  if send_req:
                      if not multiple_files:
                          return json.loads(send_req)['id']
                      else:
                          letter_id = json.loads(send_req)['id']
                  else:
                      return send_req
  
              if idx > 0:
                  if idx == last:
                      self._make_patch_request('letters/{}'.format(letter_id), data={'incomplete': False}, files=files)
                  else:
                      self._make_patch_request('letters/{}'.format(letter_id), data={'incomplete': True}, files=files)
  
              idx += 1
          return letter_id