If it is a Carrier Document, upload that in Settings> Insurance Carriers> Documents.
If it's a lead/client specific file, upload that in their Files tab.
To add in general files, go to Settings> Weblinks & Email Attachments. Click Add New File and upload.
Once the files have been uploaded, they will be displayed as options to attach in Send Email from the lead/client files and in the template creator, below the body of the message. (Screenshots below). You can also quickly access Files in the Links & Files tab on the left navigation bar.