Hello. I also have "External resource e6 has unknown mime type" with some books.
However, I find that if I open the epub in the Kindle Previewer directly, save the .kpf file, run
Code:
calibre-debug -r "KFX Output" -- input.kpf output.kfx
, then it converted without any problem.
So I am thinking that maybe it has something to do with preprocessing done by calibre. Any idea?