diff --git a/synapse/rest/media/v1/filepath.py b/synapse/rest/media/v1/filepath.py
index 43d0eea00..6923a3fbd 100644
--- a/synapse/rest/media/v1/filepath.py
+++ b/synapse/rest/media/v1/filepath.py
@@ -35,7 +35,7 @@ def _wrap_in_base_path(func):
 class MediaFilePaths(object):
     """Describes where files are stored on disk.
 
-    Most of the function have a `*_rel` variant which returns a file path that
+    Most of the functions have a `*_rel` variant which returns a file path that
     is relative to the base media store path. This is mainly used when we want
     to write to the backup media store (when one is configured)
     """
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index a3288c9cc..e986e855a 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -343,6 +343,9 @@ class PreviewUrlResource(Resource):
     def _expire_url_cache_data(self):
         """Clean up expired url cache content, media and thumbnails.
         """
+
+        # TODO: Delete from backup media store
+
         now = self.clock.time_msec()
 
         # First we delete expired url cache entries