I just send this to apple feedback, maybe others are interested in this and should provided feedback as well...
When I use the "attach" button in mail, there is a checkbox that allows the user to make the attachment windows friendly. I think that this shouldn't be an option, mail.app should always produce windows friendly attachments without the user having to specify it in particular. Also, because when one drags and drops a file into a new message the ability to create a windows friendly document is not available.
Please make creating a windows friendly attachment the standard, not just an option.