>>>I tried just using ASMA, but that did not work.
This should work. Did you enable the check box for filename and directory under ASMA.
You don't even need dynamic configurationbean for this.
Once you assign the values in the mapping, go to advanced tab and enable ASMA for FileName and Directory.
Also make sure that the namespace matches with what you given during the message mapping "http://sap.com/xi/XI/System/File "