Interesting clue for the missing attachments in Mail problem

[LeicaM13]

I'm still troubled by not receiving certain types of attachments in my Mac Mail app. They are there. I can read them in another reader, such as through my browser to Comcast's webpage. If I send one, it's not there later. If I receive some, they are not there. Sometimes the title to .jpg images appear. The only difference I can glean from the header is this difference: I cannot receive those when the Content Type is this: Content-Type: multipart/alternative; boundary. However I can receive the ones when the Content-Type is: Content-Type: multipart/mixed; boundary.

I've checked the attachments options: Always send Windows friendly attachments, and Always insert Attachments at End of Message.

I'm running OS 10.6.3, Snow Leopard.

If you can give me any suggestions on how to fix this in Mail please do.

Thanks