[BUGFIX] Fix fileinfo property mapping in ResourceStorage::setFileContents

Fixes the handling of the mismatch between the file info property
"mimetype" and the file object property "mime_type" by using the
exposed API methods.

Fixes: #49386
Releases: 6.2, 6.1, 6.0
Change-Id: If948de2a8bead340ae97097b3e4e391a27296362
Reviewed-on: https://review.typo3.org/21634
Reviewed-by: Steffen Ritter
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind