Media type = image/webp, as per
https://en.wikipedia.org/wiki/WebP
As webp offers almost double quality for half the weight than jpg, I am thinking to change the covers of my personal books to that format, besides using them inside.
Inside is not a problem for me, nothwistanding the wrong media type (I read in calibre viewer, and have checked already).
Cover, however, got messed when inserting it in sigil (by hand)...