File proxy power-up is used to proxy any file through the server Google Tag Manager. This feature helps to load files from your domain instead of using third-party domains. 

To set up file proxy power-up:

1. Add the URL of the file that you want to proxy. 
2. Specify the sGTM path that should be used to proxy the file through the tagging server URL. For example, if your tagging server URL sgtm.example.com, and the sGTM path is /file.js. Then the file will be loaded from sgtm.example.com/file.js. 
3. Set maximum cache age.