Confusion on how to add attachments to transactional emails

[jumpsplat120]

I'm currently trying to attach a pdf to a transactional email. Looking at the docs, it says that it takes a Base64 encoded chunk data of the attachment generated on the fly, but the actual type is a Blob. I've actually tried both a Blob and a base64 string, but while the email does send, it doesn't have the attachments.

What's the correct way to add an attachment?