In reply to Otto.

That's probably because they didn't realise it was a problem. I think I saw that blog post but ignored it as it seemed irrelevant since my screenshots work just fine on WordPress.org.

This only seems to be a problem when trying to view the screenshots on their own. Or perhaps this is a problem in some browsers which don't handle the correct mime-type?

At any rate, I'll eventually fix those mime-types on my own plugins, but I'm not in a hurry to do that since I can't see any real practical problem with them the way they are now.