Hello Krileon,
You don't substitute it in. It should just be attached to the email (try inviting one of your other email accounts to test).
Yes, that's what I did.
I just rechecked without any tag : I confirm that I have not
been able to make it work ( nor declaring a local path in the
"attachement" field, nor declaring a url starting with or
without "http://"
I haven't found anything "strange" in the logs neither, at least
nothing about the file that I try to attach to the invite message.
Best