[TASK] Code cleanup EXT:backend/Classes/File/*.php

- add missing return values
- add missing phpdoc comments
- remove unused variable assingments
- add getter for LanguageService
- add getter for BackendUser

Resolves: #65498
Releases: master
Change-Id: I0314fa03b9ac885c46e281a167d04f060fb10617
Reviewed-on: http://review.typo3.org/37513


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>