[FIX] website_form: display attachment on form web page

When submitting a ticket, if the user adds an attachement, he will not
be able to see it on the ticket web page.

To reproduce the error:
(Need helpdesk,contacts)
1. Go to the settings of a Helpdesk Team (e.g., "Customer Care")
2. Enable "Website Form", Refresh
3. Go on Helpdesk web page and Publish the form
4. Create a partner P and grant him the portal access
5. Sign in with P
6. Submit a ticket with one attachment
7. Consult the ticket's web page

Error: in "Message and communication history", OdooBot's message with
the attachment is not displayed. However, when consulting the
same page with the admin account, this message is considered as
"Published". Thus, P should see the message and the attachment.

When getting the messages, if the user is not an employee, the domain is
restricted:
https://github.com/odoo/odoo/blob/e92ac60a75462dfd830254a3d0dd5d7effe1528e/addons/portal/controllers/mail.py#L128-L130
with
https://github.com/odoo/odoo/blob/cc32b7d1b396a42dd4575c681ceb422f99b6015a/addons/portal/models/mail_message.py#L30-L32


Problem is that, when creating the message with the attachement,
`subtype_id` is not defined.

OPW-2440069

closes odoo/odoo#66841

Signed-off-by: Adrien Widart <adwid@users.noreply.github.com>