Attachments aren't supported by mailto: according to the spec (RFC 2368
http://www.faqs.org/rfcs/rfc2368.html).
Though Neil Crosby,
http://www.freevbcode.com/ShowCode.asp?ID=5213, insists that attachment can be sent.
ShellExecute invokes default mailer. So, I think, it depends on the mailer software, whether it supports mailto functionality and to what extent.
#define SW_HIDE 0
#define SW_SHOWNORMAL 1
#define SW_SHOWMINIMIZED 2
#define SW_SHOWMAXIMIZED 3
#define SW_SHOWNOACTIVATE 4
#define SW_SHOW 5
#define SW_MINIMIZE 6
#define SW_SHOWMINNOACTIVE 7
#define SW_SHOWNA 8
#define SW_RESTORE 9