In FileBackendBase/FileBackend:

In FileBackendBase/FileBackend:
* Changed concatenate to store to a specified temp FS file rather than a final storage destination. This makes it better fit the use case (chunked upload), so we can avoid extra copying around. Subclasses no longer have to implement this function now as well.
* Added extensionFromPath() helper function.
* Moved createInternal() up a bit and fixed @see comments pointing to the wrong functions.
In FSFileBackend:
* Use parent implementation of doConcatenateInternal().
In FileRepo/File:
* Added FileRepo::ALLOW_STALE and made thumbnail transforms use it.