ES Disable File Sharing in Conversations.. Where do I set that?

When my users message each other they can currently upload files. I want to turn that feature off but can't find where I originally turned it on!

I see a reference to files in User Profiles but that sounds like it relates to the 'Share what's new' stream and I definately what them to be able to share files there... just not in private conversations.

Thanks for any guidance to find this!