Actually, at least for me, Mail puts attachments in ~/Library/Mail Downloads/. Not as any sort of temp files or folders. If I click on the attachment in the header of the message, it opens in the default app for the file type. If I double click the attachment as displayed within the message body, it does the same. Never puts a copy of the file on my desktop unless I tell it to Save. Maybe it has to do with the type of mail server (POP, IMAP, etc.) or a preference somewhere?