[FIX] project: correct filename on portal attachment

Create a project visible to a portal customer, create a task and attach
an attachment. As a portal user, go to the task and download the
attachment, the downloaded filename is the numeric identifier of the
document instead of the document filename.

The `?download=true` force the download of the attachment and correctly
set the filename.

opw-2008517

closes odoo/odoo#34098

Signed-off-by: Julien Castiaux <Julien00859@users.noreply.github.com>