def _add_content_type(self, partname, content_type):
        """
        Add a content type for the part with *partname* and *content_type*,
        using a default or override as appropriate.
        """
        ext = partname.ext
        if (ext.lower(), content_type) in default_content_types:
            self._defaults[ext] = content_type
        else:
            self._overrides[partname] = content_type