1. Go to "settings" and then "campaigns".
2. Select "Email Template", and then edit on the small pencil to the right of the mail template that should contain a file.
3. In the bottom right corner, select "Attach File" on this button:
4. Now select the file you want to attach.
If you want to use merge tags in your file, this is also possible. Insert merge tags into a word file that you subsequently attach.