decision matrix for mime types:

so d = 0…15.

MIME-Type decision map
🤔		Attachment types
🤔		0:none	4:inline	8:attachment	12:both
Content	0:none	0: text/plain (empty)	4: multipart/mixed	8: multipart/mixed	12: multipart/mixed
	1:text_plain	1: text/plain	5: multipart/mixed	9: multipart/mixed	13: multipart/mixed
	2:text_html	2: text/html	6: m/rel(html, inline)	10: multipart/mixed	14: m/mix( m/rel(html, inline), att )
	3:both	3: m/a(text,html)	7: m/a( text, m/re( html, inline) )	11: m/mix( m/a(text, html), att )	15: m/mix( m/a(text, m/rel(html, inline)), att )

m/rel := multipart/related
m/mix := multipart/mixed
m/a := multipart/alternative
inline := "attachments" with Content-Disposion:inline or with name starting with "cid://"
att := all other "attachments"