[BUGFIX] Drag&Drop upload broken after PSR-7 change

Resolves: #70955
Releases: master
Change-Id: I8d4fdf1a2cf0db8cbccdc96306ba78e029da3abf
Reviewed-on: https://review.typo3.org/44225


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>