diff --git a/docker/.env.example b/docker/.env.example
index 83d975cec5..1adb07ca64 100644
--- a/docker/.env.example
+++ b/docker/.env.example
@@ -39,6 +39,12 @@ APP_WEB_URL=
 # File preview or download Url prefix.
 # used to display File preview or download Url to the front-end or as Multi-model inputs;
 # Url is signed and has expiration time.
+# Setting FILES_URL is required for file processing plugins.
+#   - For https://example.com, use FILES_URL=https://example.com
+#   - For http://example.com, use FILES_URL=http://example.com
+#   Recommendation: use a dedicated domain (e.g., https://upload.example.com).
+#   Alternatively, use http://<your-ip>:5001 or http://api:5001,
+#   ensuring port 5001 is externally accessible (see docker-compose.yaml).
 FILES_URL=
 
 # ------------------------------