I can see .html attachment symbol in messages but when I open it there is no attachment.
I think Zimbra cannot decode base64 attachments.
Did you have experience about it ?

I read these articles:

Zimlet cookbook based on JavaScript API - Zimbra :: Wiki


the thread called "zimlet for decoding base64-encoded attachments" into section "Zimbra Collaboration Suite > Zimlets" , written by gray03.

what do you think ?

If it can be useful I show the original messase:

Content-Type: application/octet-stream;
Content-Transfer-Encoding: base64
PGhlYWQ+PHRpdGxlPlJlZ2lvbmUgVG9zY2FuYSAtIFByZXNpZG VuemE8L3RpdGxlPjwvaGVhZD4