Set a data value for file field is not working

Problem/Motivation

I have a rule to download a file from an url and send it as an attachement to an email, this is working very nice. I would like also to save the file in a file field. My problem starts when I create the action "Set a data value". I select the file field for my node and when I try to select the value I cannot drill down to the file in the variable. The only option in the drill down is :owner/