code: def post_attachment(session, thread_id, attachments):
      
      files = []
      filenames = []
      for attachment in attachments:
          files.append(attachment['file'])
          filenames.append(attachment['filename'])
      message_data = {
          'attachments[]': filenames,
      }
  
      # POST /api/messages/0.1/threads/{thread_id}/messages/
      endpoint = 'threads/{}/messages'.format(thread_id)
      response = make_post_request(session, endpoint,
                                   form_data=message_data, files=files)
      json_data = response.json()
      if response.status_code == 200:
          return Message(json_data['result'])
      else:
          raise MessageNotCreatedException(message=json_data['message'],
                                           error_code=json_data['error_code'],
                                           request_id=json_data['request_id'])
docs: Add a message to a thread