Hi, I do not see this behavior with a generic OOB type Inbound action configured against sc_task table, and if I send an attachment with a reply email it is attached to the sc_task record.
Perhaps you can share details of your investigation and diagnostics?
Was the inbound message correctly matched to a 'target' record?
Does your inbound action email logs show the inbound action was sucessfully run against the email message?
Does the email log show an 'attachment entry, does the email attachment list show the attachment in the list and does it have an action of 'Attached to Target record'
Can you see the attachment in sys_attachment table?
