There is a nice feature in kde which helps identify a file by its contents. This file previewer uses the mplayerthumb package for previewing movie files however, which if used with the phonon backend it could end up eating all your systems memory, briging your computer to a halt :-/
If you seem to have a problem with a process named kio_thumbnails eating all your memory even though you have specificaly asked that files bigger than xMB (where x is lower than the system memory) shouldn't have a preview, then you should have a look at mplayerthumbsconfig. It is a small gui program that comes along with mplayerthumbs and helps you config a couple of things.
The option that you should change for now is the one labeled backend. Select mplayer and you are done. This problem should (and probably will) be corrected in the next version of kde. It affects kde4 versions till kde 4.4.3 for sure. :-/