Given a list of parts, return all of the nested file parts.

def _search_for_files(parts):
    """ Given a list of parts, return all of the nested file parts. """
    file_parts = []
    for part in parts:
        if isinstance(part, list):
            file_parts.extend(_search_for_files(part))
        elif isinstance(part, FileToken):
            file_parts.append(part)
    return file_parts