zlib plugin doesn't add "Z" flag to Maildir filename

I enabled zlib compression, so new mails are saved compressed.

Now I want to convert the old mails to gzip with the find and gzip command, but now I don't know which files are already gzip compressed by dovecot.

Shouldn't zlib_save itself add Z to the Maildir flags? As suggested in the compress guide on http://wiki2.dovecot.org/Plugins/Zlib ?

-- Tom