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).
Why does it matter? If you've ever worked in a situation where you are exchanging emails with multiple attachments, each with different data, you know how easy it is for people to not find what you were pointing them to because they looked in the wrong file. It is really useful to be able to say, "This file" and then have the file be right at the end of the line. The only problem is that you can't control how the recipient's email program will display the attachments, as you can see below, so you can only be sure that it works if they have Outlook (see images of email sent from outlook and received in gmail 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.