diff --git a/swh/core/tests/data/archives/hello.tar b/swh/core/tests/data/archives/hello.tar
new file mode 100644
index 0000000..73fafe8
Binary files /dev/null and b/swh/core/tests/data/archives/hello.tar differ
diff --git a/swh/core/tests/data/archives/hello.tar.bz2 b/swh/core/tests/data/archives/hello.tar.bz2
new file mode 100644
index 0000000..730b09f
Binary files /dev/null and b/swh/core/tests/data/archives/hello.tar.bz2 differ
diff --git a/swh/core/tests/data/archives/hello.tar.gz b/swh/core/tests/data/archives/hello.tar.gz
new file mode 100644
index 0000000..87727db
Binary files /dev/null and b/swh/core/tests/data/archives/hello.tar.gz differ
diff --git a/swh/core/tests/data/archives/hello.zip b/swh/core/tests/data/archives/hello.zip
new file mode 100644
index 0000000..e9dde2a
Binary files /dev/null and b/swh/core/tests/data/archives/hello.zip differ
diff --git a/swh/core/tests/test_tarball.py b/swh/core/tests/test_tarball.py
index 58c9fbd..725d9a6 100644
--- a/swh/core/tests/test_tarball.py
+++ b/swh/core/tests/test_tarball.py
@@ -1,119 +1,121 @@
 # Copyright (C) 2019  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import os
 import pytest
 import shutil
 
 from swh.core import tarball
 
 
 @pytest.fixture
 def prepare_shutil_state():
     """Reset any shutil modification in its current state
 
     """
     import shutil
 
     registered_formats = [f[0] for f in shutil.get_unpack_formats()]
     for format_id in tarball.ADDITIONAL_ARCHIVE_FORMATS:
         name = format_id[0]
         if name in registered_formats:
             shutil.unregister_unpack_format(name)
 
     return shutil
 
 
 def test_compress_uncompress_zip(tmp_path):
     tocompress = tmp_path / 'compressme'
     tocompress.mkdir()
 
     for i in range(10):
         fpath = tocompress / ('file%s.txt' % i)
         fpath.write_text('content of file %s' % i)
 
     zipfile = tmp_path / 'archive.zip'
     tarball.compress(str(zipfile), 'zip', str(tocompress))
 
     destdir = tmp_path / 'destdir'
     tarball.uncompress(str(zipfile), str(destdir))
 
     lsdir = sorted(x.name for x in destdir.iterdir())
     assert ['file%s.txt' % i for i in range(10)] == lsdir
 
 
 def test_compress_uncompress_tar(tmp_path):
     tocompress = tmp_path / 'compressme'
     tocompress.mkdir()
 
     for i in range(10):
         fpath = tocompress / ('file%s.txt' % i)
         fpath.write_text('content of file %s' % i)
 
     tarfile = tmp_path / 'archive.tar'
     tarball.compress(str(tarfile), 'tar', str(tocompress))
 
     destdir = tmp_path / 'destdir'
     tarball.uncompress(str(tarfile), str(destdir))
 
     lsdir = sorted(x.name for x in destdir.iterdir())
     assert ['file%s.txt' % i for i in range(10)] == lsdir
 
 
 def test_uncompress_tar_Z_failure(tmp_path, datadir):
     tarpath = os.path.join(datadir, 'archives', 'inexistent-archive.tar.Z')
 
     assert not os.path.exists(tarpath)
 
     with pytest.raises(shutil.ReadError,
                        match=f'Unable to uncompress {tarpath} to {tmp_path}'):
         tarball.unpack_tar_Z(tarpath, tmp_path)
 
 
 def test_uncompress_tar_Z(tmp_path, datadir):
     filename = 'groff-1.02.tar.Z'
     tarpath = os.path.join(datadir, 'archives', filename)
 
     assert os.path.exists(tarpath)
 
     output_directory = tarball.unpack_tar_Z(tarpath, tmp_path)
 
     expected_path = os.path.join(tmp_path, filename)
 
     assert os.path.exists(expected_path)
     assert expected_path == output_directory
     assert len(os.listdir(expected_path)) > 0
 
 
 def test_register_new_archive_formats(prepare_shutil_state):
     unpack_formats_v1 = [f[0] for f in shutil.get_unpack_formats()]
     for format_id in tarball.ADDITIONAL_ARCHIVE_FORMATS:
         assert format_id[0] not in unpack_formats_v1
 
     # when
     tarball.register_new_archive_formats()
 
     # then
     unpack_formats_v2 = [f[0] for f in shutil.get_unpack_formats()]
     for format_id in tarball.ADDITIONAL_ARCHIVE_FORMATS:
         assert format_id[0] in unpack_formats_v2
 
 
 def test_uncompress_tarpaths(tmp_path, datadir, prepare_shutil_state):
     archive_dir = os.path.join(datadir, 'archives')
     tarfiles = os.listdir(archive_dir)
     tarpaths = [os.path.join(archive_dir, tarfile) for tarfile in tarfiles]
 
-    for n, tarpath in enumerate(tarpaths, start=1):
+    unregistered_yet_tarpaths = list(
+        filter(lambda t: t.endswith('.Z'), tarpaths))
+    for tarpath in unregistered_yet_tarpaths:
         with pytest.raises(ValueError,
                            match=f'File {tarpath} is not a supported archive'):
             tarball.uncompress(tarpath, dest=tmp_path)
 
-    assert n == len(tarpaths)
-
     tarball.register_new_archive_formats()
 
     for n, tarpath in enumerate(tarpaths, start=1):
         tarball.uncompress(tarpath, dest=tmp_path)
+
+    assert n == len(tarpaths)