checks that path's extension against the Writer's supported
        extensions.  If it isn't supported, raises UnsupportedFiletypeError.

def check_extension(cls, ext):
        """checks that path's extension against the Writer's supported
        extensions.  If it isn't supported, raises UnsupportedFiletypeError."""
        if ext.startswith('.'):
            ext = ext[1:]
        if not any(ext in extension for extension in cls.supported_extensions):
            msg = ("Invalid extension for engine '{engine}': '{ext}'"
                   .format(engine=pprint_thing(cls.engine),
                           ext=pprint_thing(ext)))
            raise ValueError(msg)
        else:
            return True