[FIX] web, mail: error when adding attachments

Fix errors when adding attachment to records with read only permission:

Current behavior:
1. Error adding attachment to records with read only permission.
2. The message is not as clear as version 13.0

closes odoo/odoo#88036

Expect: Only display the message "You are not allowed to upload an attachment here." if you do not have permission to upload files.
X-original-commit: 8385f192
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>