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

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