>MailTo doesn't support attachements
This is not entirely true. It depends on the URL resolver of the client software that is decoding the MailTo URL. MailTo supports the concept and inclusion of headers. Although generally the only headers that you can guarantee the URL resolver to understand are the "subject" and "body" headers, some clients happily accept "attach" or "attachment" as one of those headers (amongst others, such as CC, and BCC).