I think this is what you’re looking for, but I’m not sure if all these functions are defined in the file_server templates (see discussion in V2 - http.handlers.templates functions in file_server browse template?) . These docs are for the templates directive, i.e. the generalized templates. That said, all the sprig functions are supported as noted.
FYI you can find the default browse template here in the code:
https://github.com/caddyserver/caddy/blob/master/modules/caddyhttp/fileserver/browsetpl.go