I'm unaware of any way to specify an attachment using mailto:. Neither RFC 2368 (
http://www.rfc-editor.org/rfc/rfc2368.txt) nor this KB article (http://support.microsoft.com/kb/287573) mention attachments.
However, it would be possible to adapt the vbscript from this article to accept the attachment location as a command line argument and include the date in the subject:
http://www.microsoft.com/technet/scriptcenter/resources/qanda/nov04/hey1129.mspx
Would this be acceptable for what you're trying to do?