Two things. First, you say you tried to access myhomeserver.changeip.net. This assumes that you didn’t put /data/media at the end of your URL. Your Caddyfile will only handle requests to that path with the proxy directive you defined.
Secondly, I may be wrong but I don’t think filebrowser supports being served from a subpath. I recommend instead writing your proxy like this:
proxy / 127.0.0.1:8080 {
transparent
}
If you need to serve other things, I recommend serving them under a different subdomain (multiple subdomains can point to the same IP, and Caddy will handle them based on which domain is in the request)