[BUGFIX] Fix thumbnail if storage is not public

Releases: 3.0
Resolves: #61544
Change-Id: Ifa2b1ee235c218545b23909c20ddb708c877f584
Reviewed-on: http://review.typo3.org/32772
Reviewed-by: Fabien Udriot <fabien.udriot@ecodev.ch>
Tested-by: Fabien Udriot <fabien.udriot@ecodev.ch>