That's because it's a chrome extension
webp is a sort of way to embed movies on webpages. Think of it as "animated gif plus".
http://en.wikipedia.org/wiki/WebP
edit: And you need Chrome to view it, I forgot to mention.
edit2: Vanilla Chrome will play webp. Since the site is the apps/plugins store for Chrome I don't think that is a issue... lol
But linking webp contents here will have that effect of only people with Chrome being able to see it.