API for unwrapping FileSystem

[yschimke]

Maybe unblocked by #1464

But given a FileSystem, I'd like to be able to unwrap it to tell it's real type. Is it FileSystem.SYSTEM under the hood? or a zip file? or resources? or S3?

[swankjesse]

I don’t want to do this because I think it’ll lead to fragile code in practice.

But I do think we should design a mechanism to add extensions to the file system API so it doesn’t have to be comprehensive always. Perhaps something like OkHttps request tags.

[swankjesse]

#1466