Map Filename to field within mapping

I have a requirement to map the filename to a field in the header of a file to file scenario and then create a new filename based upon the fields of the data structure of the header record.

I am comfortable using the variable substituion to either map the filename from sender to reciever and to use dynamic file names to create the new file name, but I am unsure how i can the read filename from the Adapter Specific Message Attributes and pass this value to a field on the outbound structure of the mapping.

Can anyone give me some pointers on how i can map the sending "Filename" to the outbound field within PI7.1 SP6?