Funny that Mozilla says MIME types are important, when I had to use their browser to get around the problem of wrong MIME type. Or perhaps worrying that the work-around may soon no longer work. Some circumstances are different, though.
However, I think the fact that a proper browser wants to save the file is exactly why the content type is application/octet-stream. Attached images are shown inline. The thumbnail itself is a link that opens the image in full size in a new window. That URL has a content type of image/<whatever>. The link below the image is probably intended for downloading the image, and therefore utilizes the trick of using application/octet-stream to prevent the browser from trying to open the image itself.