def register_extension(self, ext_in, ext_out, force=False):
        """
        Add/register a file extension.


        Args:
            ext_in (str): Extension of input files.
            ext_out (str): Extension of corresponding output files.
            force (bool): If ``force`` is set to ``True``, simply overwrite existing extensions, otherwise do nothing.
                If the ``logger`` is set, log a warning about the duplicate extension if ``force == False``.


        """
        if not force and (ext_in in self.__extensions.keys()):
            self.log_warning("Extension %s already exist, ignore redefinition." % ext_in)
            return

        self.__extensions[ext_in] = ext_out