For years I have been stumped by one of the small mysteries of Outlook: 99% of the time my emails would have their attachments in the attachment field. The other 1% of the time the attachments would be inline with the text (the two possibilities are illustrated below).
I can recall one of my consulting teams back in 2005 trying to figure out why this happens, and have wondered on many occasions why this happens. I finally came across the answer: It turns out that if the email is in HTML or plain text formal, the attachment goes in the attachment field, but if it's in RTF it goes inline.
Go back and check every email you've ever had w/ an attachment inline and you will see that they were in RTF format. Emails sent as replies to calendar items in outlook are RTF format btw, which explains why they have inline attachments.