Install Accounting, go to Configuration > Follow-up Levels. Create a new
Follow-Up Action with low due days just to make testing easier and "Join
open invoices" enabled. Go to Customers>"Follow-up Reports", select a
follow up report with action "test", click on "Send Email"

The email will be sent, without attachments. There should be a number of
attachments equal to the number of due invoices, but for security reason
the attachments are filtered in the message post processing phase.

Changing security check to avoid filtering legitimate material.

opw-2116803

closes odoo/odoo#40181

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>