If you are like me and many other internet users, you’ve probably downloaded movies with one P2P program or another.
If you did that downloading while using Nautilus and Gnome, you might have noticed that Nautilus fails to produce Thumbnails for downloaded movies, presenting the film-roll Icon instead.
What happens here, is that Nautilus attempts to generate thumbnails for files as soon as you enter the directory that contains them, when it comes to files that are only halfway-downloaded it fails for obvious reasons.
The problem is that when Nautilus fails to generate a thumbnail, that failure is noted down somewhere, and therefore Nautilus does not attempt to regenerate the thumbnail once the file is fully downloaded.