We use URL links to other attachments. It is not ideal but the only thing we have found to work so far. You do have to upload those attachments somewhere. If it is just instructions, we create a Canva presentation and use the presentation link to share the instructions. 

we also use links to link out to documents - this could technically be a link to content within a course/class in TI if it’s either free, or if they’re logged in (i.e. an article, pdf, etc) or when it’s not content housed in TI we will use our wordpress media library to upload it to and then link it within the notification. 

Attachments pose an issue for cybersecurity. Many companies will block these types of emails and it increases your risk of being flagged as spam or decreasing your deliverability rating.