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