def compression_type(self):
        """Return the latest compresion type used in this MAR.

        Returns:
            One of None, 'bz2', or 'xz'

        """
        best_compression = None
        for e in self.mardata.index.entries:
            self.fileobj.seek(e.offset)
            magic = self.fileobj.read(10)
            compression = guess_compression(magic)
            if compression == 'xz':
                best_compression = 'xz'
                break
            elif compression == 'bz2' and best_compression is None:
                best_compression = 'bz2'
        return best_compression
Return the latest compresion type used in this MAR.

        Returns:
            One of None, 'bz2', or 'xz'