docs: def is_multipart(header_dict):
      """
    Args:
      header_dict : CaseInsensitiveDict
  
    Returns:
      bool: ``True`` if ``header_dict`` has a Content-Type key (case insensitive) with
      value that begins with 'multipart'.
    """
      return (
          {k.lower(): v for k, v in header_dict.items()}
          .get('content-type', '')
          .startswith('multipart')
      )