Which version are you using?
Have you tried using Variables?
6.0.4 - im new to this so trying to figure it out.
in new name mask i have FFI_{DATEFORMAT(ddMMyyyy-HHmm).*
post process... is ticked with the following in it :
{STRING(Replace|{NEWNAME(FFI_{DATEFORMAT(ddMMyyyy-HHmm)})}|oldValue|newValue)}