Hi,
I'm not sure what's your PI version is, but I'm thinking of the below options..
See Step 8: Keep Attachments on Sender HTTP adapter
https://help.sap.com/saphelp_nw73ehp1/helpdata/en/03/e2d70a626342feaef7f2f9f04a1776/content.htm
Unzip the Payload on the Receiver adapter
Working with the PayloadZipBean module of the XI Adapter Framework
Handle the attachments in Proxy (ECC)
PS: I'm just throwing out the ideas here. Which you can explore and fit according to your needs.
Thanks,
Justin.