zimcheck: check known/expected metadata mimetype #474
Description
As mentioned by @rgaudin in openzim/mwoffliner#2470, it seems that all known/expected metadata (but illustrations) should have text/plain or text/plain;charset=UTF-8 mimetype.
It looks like zimcheck is not checking this at all.
Probably worth to add. Not sure if it should be an error or a warning.