Ever run into renamed gifs on the internet?

I've seen a few times where it seemed images with a jpg extension seem to animate. It's always a subtle animation so you can really be sure. As a test I renamed a gif as a jpg on my computer and opened it in infranview and the picture still animated even tho it was a jpg. What a dirty trick? Funny that the image still works when it's been renamed like that.

 

Its because some programs look at the actual file to work out the file type, rather than looking at the file extension.

 

I have and it's annoying. But nowhere near as concerning as files with double file extensions.