dealing with html email programatically (was: getting body from message-entity programatically (was: get subject and from on wl-biff-notify-hook function))

Hi Kazuhiro San.

Now I have a new issue when getting body on plain-text when email text
is in html (probably there is a function for converting it to plain text as
wl showns html-email as plain-text-email).

I have identified three cases until now (when watching the
notifications).

Case One (It seems the simple one):
--8<---------------cut here---------------start------------->8---
Content-Type: text/html
--8<---------------cut here---------------end--------------->8---

Case Two:
--8<---------------cut here---------------start------------->8---
Content-Type: multipart/mixed; boundary="----=_Part_5289302_1587271026.1565354401620"
--8<---------------cut here---------------end--------------->8---
On this case I think some email clients split the chunks on plain-text
or text-html accordingly.

Case Three:
--8<---------------cut here---------------start------------->8---
Content-Type: multipart/alternative; boundary="0000000000003cf854058fa6860f"
--8<---------------cut here---------------end--------------->8---
On this case I think the html content arrives as an attachment.

How wl address the html content issue for getting or rendering as
plain-text?

BR