You can't add an attachment directly to your newsletter. But you can add a link to your attachment to the newsletter. For that, first upload your attachment to the cloud, for example to Dropbox or Onedrive. After that, get the link to your file and add it to your newsletter.
To add a link to your newsletter, select the word(s) in the text area of your template that you want to use to add the link (e.g. "Click here"). Then click on the link icon in the text toolbar, as shown below.
A new window appears, where you can copy the link URL. Click "OK" and there you have it!