filename is: mime    and looks like this:
Mime-Version: 1.0
plain text
<x-html>

Mime-Version: 1.0

<x-html>

Mime-Version: 1.0

<x-html>

 sed '/Mime-Version: 1.0/{n;/^$/n;/<x-html>/s/^/=/;}' mime | sed -e :a -e '$!N;s/\n=/Content-Type\: text\/html\n/;ta' -e 'P;D'
changes the file: mime to:
Mime-Version: 1.0
plain text
<x-html>

Mime-Version: 1.0
Content-Type: text/html
<x-html>

Mime-Version: 1.0
Content-Type: text/html
<x-html>