Hi Shaibayan,
The most common way is to collect the messages via ccBPM or SAP Netveawer BPM but there is the possibility to pick up the the additional files as attachements and later acces to attachments.
Check similar discussion here Merge two files from sender file adpater without BPM
How to read attachments;
Coding Java Mapping !!!! – Points to ponder
Read attachment content in Mapping
Regards.