chore: Improve FILES_URL Configuration Comments

docker/.env.example

@@ -39,6 +39,15 @@ 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.
+#
+# Set FILES_URL based on how you access the Dify server:
+#   - For https://example.com, use FILES_URL=https://example.com
+#   - For http://example.com, use FILES_URL=http://example.com
+#
+# Recommendation: configure a dedicated domain (e.g., https://upload.example.com).
+#
+# Alternatively, use http://<your-ip>:5001 or http://api:5001,
+# ensuring that port 5001 is externally accessible (see docker-compose.yaml).
 FILES_URL=
 
 # ------------------------------