If the message is being stored with attachment in the sent-folder (This happens after the mail was sent) it is very likely that the recipients have some kind of filter or firewall in place which is removing the attachment.
You can check the /var/log/zimbra.log for the outgoing message. First search for the recipients E-Mail Address and determine the message ID.
Once you know the message ID look for a line like this containing the message ID:
If the "size=131925" is equal or greater than the size of the message in the sent folder I would believe that the message has been sent with attachment.
postfix/qmgr: AB032C44EE7: from=<firstname.lastname@example.org>, size=131925, nrcpt=1 (queue active)