Password of filebrowser is auto-generated but somehow nobody on TrueNAS community seems to know that. You have to check docker logs.
sudo docker ps -a
sudo docket logs [filebrowser container id]
Warning: filebrowser.db can’t be found. Initialing in /config/
Using database: /config/filebrowser.db
Using config file: /config/settings.json
Performing quick setup
User ‘admin’ initialized with randomly generated password: XXXXXXXXXXXXXX