If you want the file as an attachment, then you can only include it from a location which doesn’t need separate authentication (as the Snap can only have one set of credentials, and those are used for the SMTP server).
If you want to take the content as-is and drop it in the email, then in your input view to the EMail Sender Snap, pass the content from the file in as a variable, and have that in the body, something like:
