docs: def default_partfactory(part_number=None, content_length=None,
                          content_type=None, content_md5=None):
      """Get default part factory.
  
      :param part_number: The part number. (Default: ``None``)
      :param content_length: The content length. (Default: ``None``)
      :param content_type: The HTTP Content-Type. (Default: ``None``)
      :param content_md5: The content MD5. (Default: ``None``)
      :returns: The content length, the part number, the stream, the content
          type, MD5 of the content.
      """
      return content_length, part_number, request.stream, content_type, \
          content_md5, None