mailutils 3.5-4 in debian buster not working with as expected anymore

Hello everybody,

I upgraded Debian squeeze to buster and changed our mailutils version 
and the mailing scripts I have for some automation stopped working.

I can not get Content-Disposition set to inline for the text/plain part.

I want to have an mail with a plain/text message shown inline (not as 
attachment) and an PDF file as attachment.

We are using Mozilla Thunderbird 78.11.0-1~deb10u1 as mail client for 
showing the messages.

The first example is what I used to have that used to work, but it does 
not show the text any-more because it sets the Content-Disposition to 
attachment.. for the plain/text.

[email protected]
NAMEWITHHOUSENUMBER=1234AB1
LASTNAME="de Jong"
CONTENT="Geachte heer en/of mevrouw $LASTNAME,\n\nHartelijk dank voor uw 
aanvraag voor organization-advies.\n"


echo -e "$CONTENT" | mail.mailutils  -s "organization-advies: 
($NAMEWITHHOUSENUMBER)" -A /root/organization-flyer.pdf -a "BCC: 
[email protected]" -a "From: organization B.V. 
<[email protected]>" "$EMAIL"

# wrong for plain test part...
# Content-Type: application/octet-stream
# Content-Disposition: attachment
# Content-Transfer-Encoding: 8bit
echo -e "$CONTENT" | mail.mailutils --subject="organization-advies: 
($NAMEWITHHOUSENUMBER)" --content-type=application/octet-stream 
--content-filename=organization-flyer.pdf 
--attach=/root/Helmwijk-flyer.pdf -a "BCC: [email protected]" -a 
"From: organization B.V. <[email protected]>" "$EMAIL"

# creates a three part message all with the wrong Content-Disposition: 
attachment
emily:~# echo -e "$CONTENT" | mail.mailutils 
--subject="organization-advies: ($NAMEWITHHOUSENUMBER)" 
--content-type=text/plain --attach-fd=0 
--content-type=application/octet-stream 
--content-filename=organization-flyer.pdf 
--attach=/root/organization-flyer.pdf -a "BCC: 
[email protected]" -a "From: organization B.V. 
<[email protected]>" "$EMAIL"

# Segmentation fault
echo -e "$CONTENT" | mail.mailutils --subject="organization-advies: 
($NAMEWITHHOUSENUMBER)" --content-type=application/octet-stream 
--content-filename=organization-flyer.pdf 
--attach=/root/Helmwijk-flyer.pdf -a "BCC: [email protected]" -a 
"From: organization B.V. <[email protected]>" "$EMAIL" --alternative

What to do to create my plain/text incline message with attached pdf 
document?

Kind regards,

Jelle de Jong