diff --git a/dendrite-config.yaml b/dendrite-config.yaml
index 1edb026f..0ea584aa 100644
--- a/dendrite-config.yaml
+++ b/dendrite-config.yaml
@@ -248,7 +248,8 @@ media_api:
   base_path: ./media_store
 
   # The maximum allowed file size (in bytes) for media uploads to this homeserver
-  # (0 = unlimited).
+  # (0 = unlimited). If using a reverse proxy, ensure it allows requests at
+  # least this large (e.g. client_max_body_size in nginx.)
   max_file_size_bytes: 10485760
 
   # Whether to dynamically generate thumbnails if needed.