I was having this same problem today, and I figured out what is happening.
When you send an attachment to Microsoft Outlook, the way that it handles rich text causes issues when you put the attachment icon in the body of the email.
I noticed that it only happens with non-image files. So .doc, .docx, etc. It doesn't cause problems when attaching .jpg or .pdf.
Any text that is located AFTER the icon you've attached is converted to ATT00001.txt when opened in Outlook. (and maybe other programs, I'm not sure)
The way to fix this is either:
1) drag or place the icon to the very end of the email, (including after the signature) OR
2) Go to the "Edit->Attachment" menu and check "Insert attachments at end of message."
Note, with option 2, you can set it as your default, thus not worrying about it.
The only issue is that when sending messages to other mac users, you can't have attachment icons show up in the middle of the email body.
Message was edited by: Eukaryote