Thanks to a top notch open source software developer that volunteers time to our organisation this issue was finally solved. Hopefully the following will assist others.
After taking note of this posting where jholder explains the same solution for a similar problem in an earlier version:
https://www.zimbra.com/forums/admini...e-files-2.html
Andrew Tridgell (of SAMBA/rsync fame) sent me the following info which cleared up the problem:
The problem was that you still had the mime type information loaded in
the zimbra ldap database from the non-free version of Zimbra. I used
ldapdelete to delete that old mime information like this:
ldapdelete -x -h `zmhostname` -D "uid=zimbra,cn=admins,cn=zimbra" -w `zmlocalconfig -s -m nokey zimbra_ldap_password` -r cn=mime,cn=config,cn=zimbra
then I added the correct mime information like this:
ldapmodify -x -h `zmhostname` -D "uid=zimbra,cn=admins,cn=zimbra" -w `zmlocalconfig -s -m nokey zimbra_ldap_password` -a -f /opt/zimbra/openldap-2.3.43.7z/etc/openldap/zimbra_mimehandlers.ldif
and now it seems to work fine.
The old mime information had records containing
"zimbraMimeHandlerExtension=convertd" which referred to a java class
that doesn't exist in the free version.