Here's a definitive resolution for the "winmail.dat" issue.
If you, as an Outlook user, send e-mail formatted as Rich Text (RTF), and you send it to a person whose mail client does not support RTF, then that person will receive a plain text message, and everything else - attachments included - will be wrapped up into an unreadable "winmail.dat" file.
There are two ways for Outlook users to prevent this from happening:
1.) When composing new messages, make sure to set the format to "Plain Text" (Format menu -> Plain Text).
2.) When sending to a recipient that you know has no RTF mail capabilities, modify the recipient's contact record to indicate this. Go to your address book, click once on a contact name, then click the "Properties" button. Then, click on the folder tab that says "Name," and at the bottom of the window you'll see a checkbox that says, "Send e-mail using plain text only." Check this box, save the changes, and you will be all set.
More info and links can be found in the Microsoft Support Knowledgebase:
Winmail.dat Attached to All Internet Messages Regardless of Setting on Client.
Winmail.dat and APPLICATION/MS-TNEF (Microsoft Transport Neutral Encapsulation Format) message parts are encoded in a proprietary format only Microsoft Mail and Microsoft Exchange can read. (They can indeed contain things like Excel spreadsheets and Word documents, but when they do the documents are impossible to extract.)
Since the winmail.dat and MS-TNEF message parts are useful to other MS Mail users, and since even a message apparently going elsewhere might get forwarded back to an MS Mail account, we generally suggest passing them along and letting non-MS Mail users just ignore them.