I have a couple of use-cases where what I'd really like to know when dealing with an afero filesystem - say an BasePathFS or a ReadOnlyFS - is what the underlying filesystem that's being wrapped is.

This is in effect kind of asking for a "unsafe" mode - under some circumstances, is for example the filesystem is backed by OsFs then I'd like to know so I can choose to run a different type of operation that is either more efficient or possible. In my specific case it's choosing between serving git repositories using the git binary or serving them using go-git (which is considerably slower) - but in both cases I need to discover that OsFs is what's there, and extract a path to send to the git binary to operate with (while acknowledging I might be violating some assumptions).