(fs) Path should have a method to obtain the filename extension

For some reason Path.getFileName() returns a Path, not a String. This makes it clumsy to see if the filename has a particular extension, since you need an ugly .toString() to get the string of the Path before you can call .endsWith.

It would be nicer if there was a recognition that manipulating extensions was a common operations on many path objects, and if there was API for that.