naming convension for a flat file

Hi all,

I am generating a flat file by using fm 'gui_download'.The naming convention of that flat file should be like this "al_ponumber_polineitem_labelid.pj".

the requirement is "When the po is generated the flat file should generate and the flat file naming convension should be "al_ponumber_polineitem_labelid.pj".

Need help very urgent,

Thanks