Files stored in your File Library can be attached to email templates.
First, upload the file from:
Files > Upload New File
On the upload screen, you can choose to upload the file to the the CRM Public Storage FTP or choose from your own server.
See Setting Up FTP locations.
Choose the file type as either: Image, Document, Video, Misc and select the file from your computer.
To add attachments to email templates edit a template from:
Files can only be attached to saved templates and not while creating a new one. Make sure to save your template first.
On the right side of the page, you can search for files to be attached to the email template.