[BUGFIX] Usage of deprecated returnFilemounts()

Some places in Core still use $GLOBALS['BE_USER']->returnFilemounts()

Resolves: #36405
Releases: 6.0, 6.1
Change-Id: I6252ed1191811a9813e590aa570741d31522895c
Reviewed-on: https://review.typo3.org/18513
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov
Reviewed-by: Wouter Wolters
Reviewed-by: Sebastian Fischer
Reviewed-by: Christian Weiske
Tested-by: Christian Weiske
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn