Attaching a file to e-blasts can be done by linking to the file from the message. Strictly speaking the file is not an "attachment" but a link.
This way you can track who clicked on the link to download the attachment while there is no way to track "standard" attachments.
Link to a file from a message
Highlight the link text in your message that will link to the attached file and click the Link icon in the editor toolbar.
On the link properties window click the Browse server button to select a file to link to.
If the file is not uploaded to Envoke yet you can upload it on the file at this time.
The upload tab is in the top left corner of the browser.