An email with an attached file that happens to be an HTML file is not an HTML email, and should not be rejected as one (IMHO).
What was the attachment?
In this case it was a test, to see what the behaviour would be :) The use case would include emailing design mock-ups done in HTML, etc.
I see. Including your normal email body as HTML (even with a plaintext part) is strictly prohibited and I'm not sure how to distinguish these.
attachment disposition?
Perhaps. I'll look into it.
I'm also interested in being able to attach HTML files.