I have a few users who are experiencing the following problem.
User A sends an email (from Thunderbird or apple mail) with an attachment to User B, who is using Zimbra web client.
When User B replies back to the email, his/her text response can not be seen in the body of the email. The only way to see it is by looking at the raw data. What User A will see is an embedded object in the body called "mime-attachment.jpg" or "mime-attachment.tiff"
I even have reports from one user who can't see the attachments at all and they are using the Zimbra web client, in this case User B uses zimbra web, and User A is on a standard mail client, e.g. Thunderbird, Apple Mail.