Hi Jack,
You’re very welcome for the information! At this time, our documentation does not specify that we support AWS S3 protocol within the Attachment field in the Email Sender Snap.
https://docs-snaplogic.atlassian.net/wiki/spaces/SD/pages/1438208/Email+Sender
I am not sure if such an enhancement is in the roadmap, but I could connect you with the right folks who can provide more information on this.
In the meantime, the example I described (parsing the file contents and inserting that to the email body) could be used. Please let me know if you would like to get in touch with our PM team.
Thank you for your time and patience.
-Charlie