Attachments are referenced at server by the attname (2.5) or filereference (12+).
Inside the message, cid is used
I'm working in adding support ASV12+ to the IMAP backend.
Currently it's working pretty fine (MIME format for iOS, HTML/TXT format for Android).
If interested, you can get a copy here
(I'm using Z-Push 2.0.4 in that branch) https://github.com/fmbiete/PHP-Push-2/tree/imap_asv12