1. Go to Setup.
2. Under “Administer,” click Communication Templates | E-mail Templates.
3. Click on the name of the e-mail template that you would like to attach a file to.
4. Click the Attach File button.
5. Complete the three steps and click the Done button.

In Lightning Experience:

1. Go to any object that has Email publisher available.
2. Go to the Email tab.
3. Select the Email template that you wish to add an attachment to by clicking on the InsertCreate or update Template button.
4. Select Insert Template.
5. Click the Attach file button and select the file you wish to attach to the template.
6. If you would like to save this template with the attachment for future use, click Insert, create or update Template and select Save Template.

At this point, the file will be attached to the email template, and can be used the next time you wish to send an e-mail with that attachment.

Please note the following restrictions when using attachments on e-mail templates:

1. If you are sending an e-mail to a single Contact, Lead or Case Contact, the file you attached to the template will be sent along with the e-mail.
2. If you are sending a mass e-mail, the attachment will be sent as a link attached to the message, and also embedded in the body of the e-mail. To view the file, click the link and this will open the file on your computer.

You are still able to attach files from your computer directly to individual e-mails sent from Salesforce using the ‘attach file’ button on the e-mail form.