[Jellby]

I think I probably missed the fact that advzip could be messing with the mimetype file. So far, I haven't had any problems, either in an actual reader (Kobo) or in calibre (including the "check book" option in the editor), so it could be that advzip is actually not touching the mimetype.

Anyway, the file is so small that even adding it with -X9 results in it being stored without compression.