[LibreOffice 18.104.22.168 in Ubuntu 15.10]
In mail merge to e-mail, the main document may be included as text or HTML in the body of the message, or attached as ODF, MS Word, etc. But in the last case, that attachment is still the body of the message in a way (the real content). If I am not mistaken, it is not possible to add other attachments as annex to the main document. But this must be a frequent situation: e-mailing a personal message, with annexes.
This possibility should be added.