How Attachment In Field works?

[joebordes]

This option expects the contents of the selected field to be a path to the file you want to send. How that path is filled in is not defined.

If you update your code as of today. If you add a relation field to Documents and select the "Document File Name" the attachment on the document will be sent.

So, yes, as of today, you can do what you are asking for.