@reboot /path/to/start_file_server.sh

five minutes after reboot

@reboot sleep 300 && /path/to/start_file_server.sh

It works for normal users (does not have to be root).

Ref:-

• Go to the man page [“man 5 crontab”, https://manpages.debian.org/bullseye/cron/crontab.5.en.html ] → search for reboot