Get files from iOS/Android?

As far as I'm aware, there's no way to retrieve files from iOS and Android.
The functionality would be useful for things like media players, which need to provide the system with (potentially cached) album art.

No files, tests, or entry points are identified. Start by locating how flutter_cache_manager currently exposes cached files on iOS and Android, then clarify the expected retrieval API and platform behavior with maintainers. Done should mean media players can retrieve cached files, including album art, on both platforms.

Written by the indexing model from the issue text.