In some cases, a message will start with a multipart/related including a multipart/alternative; In that case, with the current code the multipart/related was urlized as a 'msg.0.bin' attachment, which was wrong.
Fixing this behaviour by:
1- allowing to go down two levels of multiparts
2- preventing any urlization if the part is not included in a 'multipart/mixed' parent to prevent urlizing the content of multipart/related or multipart/alternative.