several bugfixes to ckeditor file upload plugin; re #4068

git-svn-id: svn://develop.studip.de/studip/trunk@29361 c5b85792-5220-0410-ad23-ed5b6b8ad999
fixes #4190

git-svn-id: svn://develop.studip.de/studip/trunk@29362 c5b85792-5220-0410-ad23-ed5b6b8ad999