In the email composer, whether you’re using a template or not, start with your message:
From the DYNAMIC FIELDS select “File Resource”.
This will give you a new interface to upload files to Stellar’s resource management system. You can drag and drop new files in or click the “NEW” button to upload a new file. From here you can filter, select multiple files, copy their links, etc
After clicking “Send to Email Body” - those links will appear down within the email:
You can rename the TEXT for the links - but just make sure you do not change the links themselves. When you upload a file to Stellar, it saves the file in the cloud and keeps a reference to it for use in scenarios just like this.