Zimwriterfs create zim file with compressed illustration (wrong mimetype)

[mgautierfr]

Zimwriterfs add all metadata using addMetada (https://github.com/openzim/zim-tools/blob/main/src/zimwriterfs/zimwriterfs.cpp#L126). But the default mimetype in this case is text/plain;charset=utf-8 which is obviously wrong for illustration.