So I want to save a youtube video on my computer in avi format. I load the video, go to my firefox cache folder, find the video and rename it as whatever.avi and paste it to my desktop. I now click on the file and it plays in VLC with the .avi extension. But is the file actually in .avi format? When I go to google I see all these third party apps that convert from .flv to .avi, what is the point of these apps if all you have to do is rename the file with the desired extension? So when I change the extension from .avi to .flv to .mkv is that actually changing the format correctly? I would assume my mac would let me know if it wasn't.