This is a default setting that comes with the text format you choose in Microsoft Outlook, e.g. if you use HTML or Plain Text -format, your files get inserted in a separate line underneath the subject line, but when you use Rich Text -format, you'll end up with the file inserted in the body of the text.
In other words, you need to change the format in which text is displayed in your mails.
To do this in Microsoft Outlook 2003 (other versions might differ slightly, but the basic steps should be the same):
Voila, no more pesky attachments in the mail body!