How to can I backup files, not directories?

I want backup some files as /etc/bird4.conf, How to can I do it?

I have tried with (/etc/config/backups):

config generate 'generate'
        list dirs '/etc/openvpn'
        list file '/etc/bird4.conf'

But it don’t work.

— EDITED —

Ok, the most simple solution is the best solution:

config generate 'generate'
        list dirs '/etc/openvpn'
        list dirs '/etc/bird4.conf'