[FIX] website_sale_digital: limit attachment search

Before this commit, in a sales order preview, a search is performed to
search all the attachments in invoiced products or free products.

Now, the search is only performed if the product is a digital product.

opw-2172563

closes odoo/odoo#43429

X-original-commit: 2c9e7832a02d2e4e2f219e979dd8ae3cbaa77bd9
Signed-off-by: Jorge Pinna Puissant (jpp) <jpp@odoo.com>