can route attachments.office.net to another proxy ?

Hello

    we use proxy pac to route all office365 through proxy (call proxy1) .But bandwidth always full and user download the attachments file from outlook so low , we can route domain attachments.office.net to another proxy (call proxy2) and another internet to can download quickly ?

     doese attachments.office.net  contain  all the attachments in mail ?

Best Regards,

Thanks