I thought this question might get a better response in AskUbuntu, as it seems to have more to do with Ubuntu than Windows at a glance.
Let's say I have a foo.mkv file. Thumbnail previews work in both Windows 7 and Ubuntu.
When I change the filename to anything random like foo.bar or when I remove the extension itself (making it just foo), Nautilus shows thumbnails normally like if it can recognize what type of files they are - without looking at file extension.
This however, doesn't happen in Windows 7. Windows starts asking me things like which application I want to use to open that file as soon as I remove file extension (forget thumbnails...) etc.
So, How does this thumbnail preview work in Windows 7 and Ubuntu? What makes Ubuntu recognize files "out of the box" unlike Windows 7?