In reply to Otto.

This only seems to be a problem when trying to view the screenshots on their own.

Exactly, and if the screenshot covers a lot of screen area people click on it to see it clearly in its original size.

IE11 opens it correctly while Firefox and Chrome open the download dialog. It happens because the "Content-Type" HTTP response header is set to "application/octet-stream" instead of "image/png" or "image/jpeg".

If I'm viewing this plugin on a laptop screen I'd want to view the enlarged screenshot and if I have to deal with the download box each time it is irritating.