I don't remember any special use of the MimeType in Onboard and considering that Onboard does not use any file associations, removing the MimeType from the desktop entries to silence the lintian warnings should not have any adverse side effect.
I don't remember any special use of the MimeType in Onboard and considering that Onboard does not use any file associations, removing the MimeType from the desktop entries to silence the lintian warnings should not have any adverse side effect.