[BUGFIX] Follow-Up: Missing signals on adding file

Mocked driver needs to return a proper "file" on calls to addFile().

Change-Id: Iddfc2f9b7e71101ad229a513727f65f64d654d45
Releases: 6.2, 6.1, 6.0
Resolves: #50795
Reviewed-on: https://review.typo3.org/23754
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters