Revert "filebackend: cleaned up the FileBackend constructor"

Actually this messes with the implicit backend made for things like Math (when unconfigured), which uses the "new" operator.

This reverts commit 1f129a22cbc8546b89dd4b49b2f567c1f758a968.

Change-Id: I4c72c4f7c8b82e38df5496cf2b90fc9e19c40334
Merge "Revert "filebackend: cleaned up the FileBackend constructor""