| Subject: | Unexpected behaviour with all three of attachment, html, and text parts |
When generating an email with both html and text parts the underlying
structure is multipart/alternative containing the two sections.
However when generating one with html, text and an attachment the
structure is multipart/mixed and then the three parts at the same level.
I would have expected the top level to be multipart/mixed, then the
second level to contain multipart/alternative and the attachment (with
the multipart/alternative section containing the text and html parts)