I ran across an issue in production where a couple end users running AppleMail versions (2.1077) and (2.936) are unable to see PDF attachments to a seam mail.
Upon further investigation I've created a patch that fixes the message layout of messages which have attachments. The issue is where all attachment inline and other are added to a new multipart/related attached to the root multipart/mixed. This appears to cause issues if attachments are not inline.
Basically all messages start with a root multipart/mixed at the top level.
1. If there are no inline attachments then the message body is added to the root multipart/mixed and any attachments (non-inline) are also added to the root multipart/mixed.
2. If there are inline attachments then a multipart/related is created and added to the root multipart/mixed. The message body is added to the related as well as any inline attachments. Non inline attachments are added to the root multipart/mixed
3. if there is a text alternative to a html body then a multipart/alternative is created and added added either to the root multipart/mixed or to the multipart/related depending on if inline attachments exist.
I've tested with this current versions of Outlook, with Gmail and with current versions of AppleMail. The inline attachment shows as broken in Thunderbird, but it also shows that way in the current 2.2.0GA and 2.2.1CR1 and appears to be a bug in thunderbird.
I'm going to attach a patch and a graphic so you can visualize the layout under different scenarios.