First upload file(s) to be attached to the emails, to the folder…
On the Email > Autoresponders page, any file(s) uploaded above, show up as in the image below. When composing a new email, just click on the “add” link next to the file name to be attached to that email.
Same deal for broadcast emails. The uploaded file shows up as follows. Click on the “add” link to add to the broadcast email to be sent.