Mail added calendar item without asking permission

[davemchine]

This morning I received an email from Ms.Zinab_Hassan who wanted to discuss her inheritance with me. Included in the email were two files. They were labeled mail attachment.cs and invite.ics. I noticed Calendar popped up a reminder and it was from Ms. Zinab and sure enough there was an event in my calendar! How did Mail automatically add this item and how do I prevent it from happening in the future?

Btw, I checked Mail's preferences and I see the option to automatically add things to Calendar but mine is set to NEVER.