The Files tab is available on Workstreams for all users. Here you'll find a consolidated list of all the files that have been uploaded to Action Items in the Workstream, as well as who uploaded it, the status of the associated action item and the accompanying comment.

Upload a file to the Workstream itself by dragging and dropping a file into the green area, or clicking the upload icon.

If you're searching for a particular file in Workboard, try looking here first.