MIME type set wrong

Hi, handy plugin it seems!

One thing about the screenshots is that if you set the MIME type correctly, people can actually open the screenshots without having to download them to their computers.

More info here: http://wptavern.com/plugin-authors-how-to-avoid-download-prompts-associated-with-screenshots-on-the-wordpress-plugin-directory

https://wordpress.org/plugins/display-all-image-sizes/