Always Use Windows-Friendly Attachments -- Sending attachments in "Windows friendly" format usually makes them friendlier for Macs too. The Windows Friendly Attachments feature has nothing to do with extensions and does not add them for you. So, what does it do?
By default, Mail assumes your recipient is also a Mac user and therefore includes the resource forks (if any) of attached files. Normally a Mac user sees such attachments as a single file, whereas a Windows user sees two individual files - one containing the data fork of the file and the other containing the resource fork.
When you choose "Windows Friendly" attachments, Mail strips the resource fork so that Windows users receive just one file, not two (one of which would be unusable anyway). In most cases - at least for files created with modern applications - all the crucial parts of files are in the data fork; as long as the filename has the correct extension and they have an appropriate application, Windows users can open the file.
The term "Windows Friendly" seems to imply that using this option makes your attachments "Mac Unfriendly." Mail's documentation reinforces this worry by stating that Mac users may be unable to open files correctly if the Windows Friendly option is used. But in practice, just the opposite is frequently true. The Mac version of Eudora, for example, sometimes cannot decode perfectly ordinary Mac files, such as Word documents, if they were sent without using the Windows Friendly setting. In other words, a wiser design might have been to make "Windows Friendly" the default behavior, with an option to make attachments "Mac Friendly" on those rare occasions when you truly must.
To tell Mail to use Windows Friendly encoding for all new messages, choose Edit > Attachments > Always Send Windows Friendly Attachments. (Although this command appears on a menu, it's actually saved as a preference.) Oddly, this command is disabled when composing a new message.
You can also toggle Windows friendliness for individual messages: When you attach a file using the Attach button on the toolbar or by choosing File > Attach File (Command-Shift-A), notice the checkbox at the bottom, Use Windows Friendly Attachments. If it's selected, all the attachments for this particular message are sent in Windows Friendly format. Unfortunately, Mail offers no convenient way to toggle Windows friendliness for attachments added to your message by drag & drop or copy & paste.
