I have noticed while trying to download avi files that usually the preview won't work while they are still downloading. I can't tell if this only happens on files that end up not working or if it's just random, does any one know?

It would be nice if it was consistant enough to be able to tell, because after downloading a 700MB file and finding it doesn't work, well that's a little discouraging. Anyone have any methods for detecting bad files while downloading them?