[BUGFIX] Flash messages are not shown with AJAX requests

Due to the use of DragUploader to upload files, flash messages
are not shown immediately but after manually reloading the page.

Change-Id: Ia2e8750792bd04209b5a1f2ce0e2a63f970ba3a8
Fixes: #56561
Releases: 6.2
Reviewed-on: https://review.typo3.org/28200
Reviewed-by: Frans Saris
Tested-by: Frans Saris
Reviewed-by: Markus Klein
Tested-by: Markus Klein