diff --git a/cellxgene_gateway/items/file/fileitem_source.py b/cellxgene_gateway/items/file/fileitem_source.py
index 5a23048..4d236bc 100644
--- a/cellxgene_gateway/items/file/fileitem_source.py
+++ b/cellxgene_gateway/items/file/fileitem_source.py
@@ -51,7 +51,7 @@ class FileItemSource(ItemSource):
         return self.convert_h5ad_path_to_annotation(item.descriptor)
 
     def list_items(self, filter: str = None) -> ItemTree:
-        item_tree = self.scan_directory()
+        item_tree = self.scan_directory("" if filter is None else filter)
 
         """def get_items(dir):
             if dir.branches:
diff --git a/tests/items/file/test_fileitem_source.py b/tests/items/file/test_fileitem_source.py
index eca34ab..782fcad 100644
--- a/tests/items/file/test_fileitem_source.py
+++ b/tests/items/file/test_fileitem_source.py
@@ -1,10 +1,31 @@
 import tempfile
 import unittest
+from unittest.mock import patch
 
 from cellxgene_gateway.items.file.fileitem_source import FileItemSource
 
 
+def stub_join(path):
+    path.join = lambda x, y: x + "/" + y
+
+
 class TestFileItemSource(unittest.TestCase):
+    @patch("os.path")
+    @patch("os.listdir")
+    def test_list_items_GIVEN_no_subpath_THEN_checks_dir(self, listdir, path):
+        stub_join(path)
+        source = FileItemSource("/tmp/unittest", "local")
+        source.list_items()
+        path.exists.assert_called_once_with("/tmp/unittest/")
+
+    @patch("os.path")
+    @patch("os.listdir")
+    def test_list_items_GIVEN_subpath_THEN_checks_subpath(self, listdir, path):
+        stub_join(path)
+        source = FileItemSource("/tmp/unittest", "local")
+        source.list_items("foo")
+        path.exists.assert_called_once_with("/tmp/unittest/foo")
+
     def test_make_fileitem_from_path_GIVEN_annotation_file_THEN_name_lacks_csv(
         self,
     ):