How can I get a dynamic filename for pdf reports, using xsl:rml?

I've tried to set the attachment field for the report declaration, but with no success. Mybe since I'm not really saving the report as an attachment, but printing it as pdf.

Any help is highly appreciated.

Code:

<report 
    id="timesheet_monthly_report" 
    string="Timesheet" model="timesheet.timesheet" 
    name="timesheet.monthly.report" 
    xsl="timesheet/report/timesheet_monthly_report.xsl" 
    auto="False" 
    menu="False" 
    usage="default" 
    attachment="(object.name or '').replace('/','')">
</report>