Recently my Mom had an odd problem with Apple Mail in Leopard and adding an attachment. She opened a new mail message, addressed it, added a subject, then wanted to just simply add an image into the body of the message. She chose the image she wanted to send, however, every time she went to insert the image in the message body, it wouldn’t insert.
Turns out, my Mom had the option Always Insert Attachments at End of Message selected. Seems that if this option is selected (it’s located under the Edit -> Attachments menu) and there is no text in the message body, Apple mail will not add the attachment.
If you want to have this option selected, the simple fix is to add at least a single character to the message body.
Not sure if this is a bug or not, but it certainly doesn’t seem very Apple to me.

July 6th, 2008 at 11:32 pm
Excellent! Thank you! I had the hardest time with this and for the life of me couldn’t figure out how to fix it.
December 15th, 2008 at 10:06 pm
I spent an hour debugging this.
January 31st, 2009 at 7:21 am
It is worse than you think, unfortunately. We have seen a repeatable bug with Apple Mail where if you drop an attachment mid-way through the body of you message, when sent it occasionally deletes all body text below where the message was dropped. This doesn’t happen all the time, but is repeatable when it does happen between a particular sender and recipient.
July 13th, 2009 at 2:29 pm
This started causing problems for me in early 90, seemed to be introduced by a security update or possibly iLife Media Browser Update (just a hunch). Images couldn’t be pasted or dragndropped into a Mail message, and it was annoying to me that the default became “Insert Attachments at End of Message”.
I opened up the com.apple.mail.plist and changed the “AttachAtEnd” entry from “1″ to “0″ fixed both issues.
Hope that helps someone, even though this thread is a bit old!