Hi Rafael,
If I understand your requirement correctly, as soon as PI File Adapter hands over the file to the FTP location, it won't have any control to take back any acknowledgment / response to its Sender. The interface flow completes at that point.
Also File adapter does not support Application Acknowledgement, only support Transport Acknowledgments.
XI: Playing with the file adapter's acknowledgments
Once the partner picks up the XML file from FTP, the IDoc must be updated to status 06 (Translation OK) and, once it deliveries to final destination (another application), IDoc status must be updated in ECC to 12 (Dispatch OK).
The above is Not possible as per my knowledge.
Regards,
Nabendu.