Normally, these files don't fragment. Plus, if they're already at the end of the volume, they can't be moved there again.

Put this as the first zone of your scripts:
FileSelect
FileName("*.iso")
or FileName("*.mkv")
or FileName("*.avi")
FileActions
MoveToEndOfDisk()
FileEnd
Edit: Replaced the second ".iso" with ".avi".