Return XML form of this content types item, suitable for storage as
        ``[Content_Types].xml`` in an OPC package. Although the sequence of
        elements is not strictly significant, as an aid to testing and
        readability Default elements are sorted by extension and Override
        elements are sorted by partname.

def _element(self):
        """
        Return XML form of this content types item, suitable for storage as
        ``[Content_Types].xml`` in an OPC package. Although the sequence of
        elements is not strictly significant, as an aid to testing and
        readability Default elements are sorted by extension and Override
        elements are sorted by partname.
        """
        _types_elm = CT_Types.new()
        for ext in sorted(self._defaults.keys()):
            _types_elm.add_default(ext, self._defaults[ext])
        for partname in sorted(self._overrides.keys()):
            _types_elm.add_override(partname, self._overrides[partname])
        return _types_elm