mailto://
is the only protocol that enables opening the client's local mail application...All the other approaches will need some additional code (plugin)...
To be precise the
mailto://
protocol does not support attachments... (
https://www.ietf.org/rfc/rfc2368.txt[
^]), even some browsers do support it... It is also a big question how do you know what file - from the client computer - you want to attach?! How do you know the path?!
If you have a complex scenario of mail (and not some simple-text-to-address) it is better to create a specific mail form and send the mail from the server using
System.Net.Mail[
^]...