"now i can reduce the amount of data on my moodle"
and
"stundents sending files for examination (like presentation videos they need to send to a course)"
Would think that changing the assignments from type that requires them to upload these presentation videos, to one where they use their own space where ever (Google is still ok - they would need a Google account), and have them provide a URL to their shared video's ... shared only with teacher of the course.
Now if you are running out of space on server ... and now you want to remove what student have submitted, that's a different story!
Not so simple ... files uploaded go to moodledata/filedir/ but they do not reside there by humanly recognizable file names. And even if you were to learn how to remove them safely, what of the assignments - students submissions would show an error ... they would loose credit for uploading ... and might even have to do the assignment again under the changed sharing assignment from their end.
To stop the 'bleeding', consider new assignments where students share the url's to their videos in the same course. You might have to hide the old assignments to prevent students from uploading.
Another thought ... backup the courses with users but without the old upload assignments by making a backup of them without those assignments. Restore the course (similar name but not replacing). Add the new URL assignments. When completed from that time forward, new students go to those changed courses.
Really don't know what will work best for ya! :|
'SoS', Ken