No errors are showing in the logs.
The main cause for this behavior is due to the mail setting "Block Outgoing Email".
When this is enabled, attachments on automated and messages created from the system will not be attached to the outgoing item.
This is done to save space. As the attachments are never going anywhere, adding them into the email could cause a large increase in the database size as they continue to increase in number.
An alternative is to use the Email Address Override field to send all emails to a single address. This will allow for attachments to be added.