Apple Mail broken multipart/related MIME format

[jkbzh]

Apple Mail's MIME messages format is broken, when it is configured to send both plain text and HTML
versions of a message by means of multipart/alternative. Under this configuration, if a user sends one or more attachments in a message, Apple mail will transform the part that carries text/html into multipart/mixed and will put there the attachments. If hypermail is configured to give top priority to text/plain, those attachments won't appear. A visual description of this coming from an actual message

multipart/alternative
#alternative 1
I 2 ├─> [text/plain, quoted, utf-8, 2.3K]
#alternative 2
I 3 └─> [multipart/mixed, 7bit, 34K]
I 4 ├─> [text/html, quoted, utf-8, 0.9K]
A 5 ├─>foo.docx [applica/vnd.openxm, base64, 23K]

Here what would be expected is that Apple Mail use multipart/alternative only for the text/plain and text/html parts and make the foo.docx attachment a sibling to that multipart/alternative block.

Even when Apple Mail is configured to send plain text, whenever an attachment is sent it seems to send the message as multipart/alternative with the attachments binded to the text/html part only.

Although this is an Apple Mail bug or design decision, it would be good if hypermail had a way to display those attachments when configured to give top priority to text/plain.