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.
