Check for returned mediatype from the registry

closes #4328
https://pulp.plan.io/issues/4328

plugins/pulp_docker/plugins/importers/sync.py

@@ -279,8 +279,10 @@ def _process_manifest_list(self, manifest_list, digest, available_blobs, tag):
                 # for compatibility with older clients, try to fetch schema1 in case it is available
                 # we set the headers to False in order to get the conversion to schema1
                 manifests = self.parent.index_repository.get_manifest(tag, headers=False, tag=True)
-                manifest, digest, _ = manifests[0]
-                self._process_manifest(manifest, digest, available_blobs, tag=tag)
+                manifest, digest, content_type = manifests[0]
+                if content_type in (constants.MEDIATYPE_MANIFEST_S1,
+                                    constants.MEDIATYPE_SIGNED_MANIFEST_S1):
+                    self._process_manifest(manifest, digest, available_blobs, tag=tag)
             except IOError as e:
                 if str(e) != 'Not Found':
                     raise

plugins/test/unit/plugins/importers/test_sync.py

@@ -133,7 +133,7 @@ def test_process_manifest_list(self, from_json, mock_manifest):
                 os='linux',
                 arch='arm')]
         self.assertEqual(step.parent.available_manifests[0].manifests, expected_man)
-        self.assertEqual(mock_manifest.call_count, 3)
+        self.assertEqual(mock_manifest.call_count, 2)
         self.assertEqual(step.parent.index_repository.get_manifest.call_count, 3)
         self.assertEqual(len(step.parent.save_tags_step.tagged_manifests), 1)