extension_map is a dictioanry with 6 initial elntries: TarArchive for '.tar', TarArchive for '.tar.bz2', TarArchive for '.tar.gz',
extension_map = { '.tar' : TarArchive , '.tar.bz2' : TarArchive , '.tar.gz' : TarArchive , '.tgz' : TarArchive , '.tz2' : TarArchive , '.zip' : ZipArchive , }