Confirm what I told you about attachment in SAP. but somehow we can either use FB03/OAOR and ZZF_POSTING_FILE to attach document as a business document.
please see attached procedure for your reference.
"Please, in transactions OAOR/FB03 always store "business document" selecting business object type = "accounting document" in order we have all BO documents attached to the same folder in Documentum.
It is important to follow this procedure , if not , SAP will use temporary tables and the file won't be saved in Documentum."
Best Regards,