A structure like this is quite common:
─><no description> [multipa/related, 7bit, 12K]
├─><no description> [multipa/alternativ, 7bit, 9.0K]
│ ├─><no description> [text/plain, quoted, iso-8859-1, 3.7K]
│ └─><no description> [text/html, quoted, iso-8859-1, 4.9K]
└─>2b0063.jpg [image/jpeg, base64, 3.3K]
Here the main content of multipart/related isn't the html part, but a
multipart/alternative containing the html part and a text part. The html
part still needs access to the onther content of the multipart/related
part, so we need to pass this through.