Here's what you can do instead:
If it is a brochure or catalog, upload your document to your website and place a link in the email template.
For example, in the email template, insert the text (without quotes):
"To download our 2015 product brochure, click on the following link:
When read by your recipient, this text will be transformed into a clickable link, and will allow your recipient to download the file.
If it is a contract to sign, once you have generated the email, attach the file directly from your email software.