code: def mimeType(self):
          
          if self.mime_type:
              # Supposed validated by the factory
              return self.mime_type
          for pattern, mime_type in self._extpattern_to_mime.items():
              if fnmatch.fnmatch(self.filename, pattern):
                  return mime_type
docs: The official MIME type for this document, guessed from the extensions
          of the :py:attr:`openxmllib.document.Document.filename` attribute, as
          opposed to the :py:attr:`openxmllib.document.Document.mime_type`
          attribute.
  
          :return: ``application/xxx`` for this file