Can't create tab from Word or Excel doc in shared channel

I could live with that - it would be a minor inconvenience if we had the ability to add a tab via the other method and base it on a Word or Excel document. I watched a video demonstration from Microsoft which suggests we should be able to do this: https://mybuild.microsoft.com/en-US/sessions/4d84d73c-08de-4f56-990b-2a73b2037df1 - fast forward to 8 mins 35. It's about sharing specific apps, but you can see from the demo that he clicks the Plus icon to create a new tab, and that Word and Excel are among the options.