<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_filter_by_extension(self, extensions, filter_type=DefaultFilterType):
 """ Add a files filter by extensions to this iterator. :param extensions: single extension or list of extensions to filter by. """

        self.add_filter(FilterExtension(extensions), filter_type)
        return self