I am using s-nail to send emails via SMTP by a bash script. Here is an example:
cat data.html | s-nail -v \
-s "Subject Text" \
-r "reply@abc.de" \
-S smtp=smtp.abc.de:587 \
-S smtp-use-starttls \
-S smtp-auth=login \
-S smtp-auth-user="admin@abc.de" \
-S smtp-auth-password="password1234" \
-a foo.img \
foo@bar.com
By default the content type is being set to Text.
How to set the Content-Type to html?
A little late but here you go
-M "text/html"