[FIX] mail: fixes the download url
[FIX] mail: fixes the download url

Following the changes to the document viewer to make it more flexible
for use in documents (89f20d2c297d99d4eb0ded8be78fbf86b8ec45d3), the download route wasn't changed accordingly.

This commit fixes this issue so the download route is based on
the model of the viewer.

closes odoo/odoo#32207

Signed-off-by: Alexandre Kühn (aku) <22220c18caede9806cb42c8e2b41136d711673bf@odoo.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo