[BUGFIX] Uncaught Exception when accessing "Filelist" module in Windows

An Exception is thrown when files are found directly in fileadmin folder.
Because of a backslash a false identifier is generated and file cannot be
found anymore.

Change-Id: I6c9e8b56654dfb44ec33f697219b5299883e82c5
Fixes: #39130
Releases: 6.0
Reviewed-on: http://review.typo3.org/14514
Reviewed-by: Oliver Hader
Reviewed-by: Wouter Wolters
Reviewed-by: Simon Schaufelberger
Tested-by: Simon Schaufelberger
Tested-by: Oliver Hader