One way to do this is this:
Click on the Reply button
-> That brings up an editor. Click on "Use advanced editor" at top right of the editor window
If you start by creating a new thread, aka "Actions" -> Create -> Discussions, then the enabled editor already has the editor in this state
-> That enables the "Attach" action at the bottom right of the editor window
-> Upload using this attach action
We will try to make this easier to do and to share.