AEM Forms: how to send email as HTML content type?

[wizard04wsu]

I have a form with the submit action set to "Send email" and the email template path set to a .txt file under /libs/fd/dashboard/templates/email. The email is always sent with the content type text/plain instead of text/html. For the contents of the template, I have tried multiple things: copying that of htmlEmailTemplate.txt, editing that to include only a simple test <div>, replacing everything with a full HTML page (starting with DOCTYPE), etcetera. I'm using AEM 6.5.13.