Rework raw file http header logic (#20484)

Rework raw file http header logic (#20484)

- Always respect the user's configured mime type map
- Allow more types like image/pdf/video/audio to serve with correct content-type
- Shorten cache duration of raw files to 5 minutes, matching GitHub
- Don't set `content-disposition: attachment`, let the browser decide whether it wants to download or display a file directly
- Implement rfc5987 for filenames, remove previous hack. Confirmed it working in Safari.
- Make PDF attachment work in Safari by removing `sandbox` attribute.

This change will make a lot more file types open directly in browser now. Logic should generally be more readable than before with less `if` nesting and such.

Replaces: https://github.com/go-gitea/gitea/pull/20460
Replaces: https://github.com/go-gitea/gitea/pull/20455
Fixes: https://github.com/go-gitea/gitea/issues/20404