[FIX] website_sale_digital: fix download attachement Py3

Before this commit, an Error 500 was thrown to the user upon downloading an attachement
It was due to two things:
- wrong return on a function
- the filename that http.send_file takes shouldn't be a string, but bytes in Py3

addons/website_sale_digital/controllers/main.py

@@ -101,6 +101,6 @@ class WebsiteSaleDigital(CustomerPortal):
                 return request.not_found()
         elif attachment["datas"]:
             data = io.BytesIO(base64.standard_b64decode(attachment["datas"]))
-            return http.send_file(data, filename=attachment['name'].encode('utf-8'), as_attachment=True)
+            return http.send_file(data, filename=attachment['name'], as_attachment=True)
         else:
             return request.not_found()

addons/website_sale_digital/models/account_invoice.py

@@ -25,4 +25,4 @@ class AccountInvoiceLine(models.Model):
 
         # I only want product_ids, but search_read insists in giving me a list of
         # (product_id: <id>, name: <product code> <template_name> <attributes>)
-        return purchases.mapped('product_id').ids
+        return [line['product_id'][0] for line in purchases]