Our system does not offer the ability to include attachments to your mailings. Most ISP's and email providers do not like receiving very large email attachments in bulk, and often block servers that send them. Attaching files would also cause the email blast to take much longer to be sent out.
However, HTML messages largely eliminate the need for attaching messages to emails. You can upload the file somewhere on the internet, and simply create an HTML link in your message. This will allow the recipient to click on the link and download the file.