CONTENT-type for sending HTML attachments

Hi guys. Im using the HTML Mime Mail <http://www.phpguru.org/static/mime.mail.html> class from phpguru.org. Anyway I've written a few scripts that use the class to attach files to it. These files are HTML reports but Im not sure what MIME content-type to use. The content type I use to for the attachments is text/html. But Im not sure that the content type for the email itself should be plain/text as this just displays the HTML attachment as garbage in the body text. Please could anyone advise, what the content type should be. Thanks, Kind regards Angelo