Added the filesystem accessor to a File class.

[robertpustulka]

Turns out I need to access the file's filesystem. So there's an accessor. Would be happy if you merged it :)

---

This change is 

[akerouanton]

@robertpustulka Can you explain your use case?

[robertpustulka]

I wanted to detect file source while working with many File objects coming from different filesystems.

Later I dropped that idea in my app, so I don't need that accessor, but you could still merge it and make it useful for somebody.