Comment 1 for bug 1787690

Possibly the message does not have a text/html part. Have you tried the following in pass_mime_types:

multipart
text/html
text/plain

The fact that the Content-Type: header is not folded with a CR and whitespace following the semicolon is not an issue.

If adding text/plain to pass_mime_types doesn't fix this, please post a complete raw message. You can elide the actual MIME part contents, but we need to see all the Content-Type: headers and part boundaries.