File Manager has been gone from Caddy for some time - removed during the later stages of v1’s development. It’s no longer a part of either version of Caddy:
File Browser’s hugo and jekyll support was removed even earlier than that:
In v1, you used to be able to have Caddy run processes in the foreground or background on certain events (like Caddy starting up or reloading), so you could still run hugo or jekyll as well as File Manager automatically and then proxy to File Manager in the background.
There’s nothing in the Caddyfile like that in v2 at the moment, but I think there were plans in the long run for the apps JSON configuration to be able to support running arbitrary programs in the background like some kind of process supervisor, which would be pretty neat. No timeline on that one, though, as far as I’m aware.