Questions about open location "mailto:" command

I’m trying to write a script that will work with the users default mail program that simply opens a new message window and inserts the subject- easy enough using mailto. Question is how do I specify an attachment? Also wondering how far back (OS9?) this command is supported. Thanks for the info!!

Mailto links have been standard in the OS for years - at least Mac OS 8.x

However, there’s no support for attachments. There is barely support for subject and message bodies. If you want to auto-create complex messages that include attachments you’ll need to target a specific mail app.