SendMailHelper start all mail with a multipart/mixed part and inside a multipart/alternative then if there is any message place their part in the multipart/alternative and if there is attachments place them after the multipart/alternative in the multipart/mixed

This cause client like Thunderbird to believe there is an attachment and display an icon before open the mail.

From observing mails in my mailbox, it seems the right behaviour should be to add the multipart/xxxx if needed.
So a mail with only a plain text content would only have one part with MIME text/plain
if there an html alternative then put both the text/plain and the text/html in a multipart/alternative
only if there is an attachment the text part or the alternative part should be placed in a multipart/mixed

The most visible impact the graphic but mail are touchy and malformed mail is the level 0 of anti spam filter !