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

def mimeType(self):
        """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
        """
        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