An error occurred while saving the commentAnonymous commented
I just ran across this issue. The ability to add a file is available on the browser version of Planner is possible, yet it's not possible using the Planner plugin in Teams. Though you can get the full web version to load in Teams by adding it as a website link. This feels like a bug more than anything that you can't include files this way.
This feature remains on the backlog. We will update you when we have status changes to share.