Add Attachment to Rules Email - Drupal 8 I am trying to set up a simple rule in D8 that when a new node is created, an email is sent out that includes an attachment from the file system.

How can I accomplish this?
Drupal core does not provide a way to send HTML e-mail or to send e-mail with attachments. Thus, you will have to use a separate module which has this functionality AND which integrates with Rules. One such module is Mime Mail. Mime Mail does have a D8 port, but it's not complete. When it is complete, Rules in D8 is ready and able to use that functionality.