I know you can click on the ics link on the main indico page to download an ics file but I was really wanting to attach it to the registration confirmation email - is this easily done? (on 2.1.11)
This will be possible in Indico 3.0:
You need to update! 2.1 is no longer supported (neither is 2.2), and the latest release (2.3.4) contains security fixes.