revert bad commit [26013]

git-svn-id: svn://develop.studip.de/studip/trunk@26014 c5b85792-5220-0410-ad23-ed5b6b8ad999
prevent xss on file upload, fixes #3365

git-svn-id: svn://develop.studip.de/studip/trunk@26015 c5b85792-5220-0410-ad23-ed5b6b8ad999