AS2 Receiver - set filename

Dear experts,

I have a scenario SAP ERP IDoc --> SAP PO --> AS2 Receiver.

The AS2 receiver is our external EDI provider and wants a specific filename.
I set up a java map that sets the filename in the Dynamic Configuration:

In the AS2 receiver channel I ticked "Respect parameters":

But in the message log, the File Name is still the Message ID, it is not set to the Filename I set in the dynamic Config:

When the AS2 message hits our external EDI partner, I can see that SAP PO message ID in the Source filename field:

Does anyone know how I can set my filename from the Dynamic Config to the actual File Name?

Thank you,
Peter