def METHOD_NAME(self) -> Optional[List[Path]]:
    """ Return the --osm-file argument as a list of Paths or None
        if no argument was given. The function also checks if the files
        exist and raises a UsageError if one cannot be found.
    """
    if not self.osm_file:
        return None
    files = [Path(f) for f in self.osm_file]
    for fname in files:
        if not fname.is_file():
            LOG.fatal("OSM file '%s' does not exist.", fname)
            raise UsageError('Cannot access file.')
    return files