[FIX] Fixes bugs in file implementation

Fixes file deletion, moving and getting file size
operations. Adds happy path tests for all operations.

Change-Id: I4b68a32fd460ca0a3b5d76c2563c84830b49a9c0
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/2032
Tested-by: Jenkins CI
Reviewed-by: Holger Schmeisky <google@schmeisky.com>