How to attach files to an email template?

You cannot attach a file directly to a Simple CRM email template. This is a technological limitation inherent in the standard "mailto" protocol.

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:
http://www.site-de-la-societe.com/plaquette_produits_2015.pdf"

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.