>.jpg
>animated
How?

Because it's a GIF. you could change the file extension to .snarflebloop for all your web browser cares. Try downloading the file and opening it in a text editor(even notepad) and you'll see that the header information at the very beginning of the file starts with the letters GIF. That's all image-displaying programs will care about. For jpegs, refer to this: http://en.wikipedia.org/wiki/JPEG_File_Interchange_Format#File_format_structure