There's not any sort of tag for that. You probably have a number of options - change AbstractFileGateway.java to add it, create an aspect that intercepts AbstractFileGateway.processingComplete, or have another process which alters .ok messages by appending the datetimestamp. There's probably other options too - those are the ones I can think of just off the top of my head.
OK. Thank you for the suggestions.
Did you look at org.jboss.soa.esb.actions.naming.FileNameGeneratorAction and associated org.jboss.soa.esb.actions.naming.strategy.AddTimestampNamingStrategy?