Supporting Android 5.0+ is impressive, though there was another file manager supporting Android 1.0+ discussed recently https://news.ycombinator.com/item?id=38789958 (I'm impressed by both, not an Android developer, curious how one ensures compatibility with ancient versions; actual testing, a compatibility linter, other?)
I also enjoy Material Files' README, particularly https://github.com/zhanghai/MaterialFiles#why-material-files (a clear articulation of "Why" is always informative) and https://github.com/zhanghai/MaterialFiles#inclusion-in-custo... (clear guidance to upstreams seems relatively uncommon, helps shed light on ecosystem dynamics otherwise invisible to me anyway).