You can share files in the Workspaces within the WebRoom by clicking the button. Open the file in the pop-up window and it automatically appears in the Workspaces.
Click the filename to open it within the room or click the icon to download the file.
IMPORTANT: Synchronized browsing (that files open up for others too when you open them within the room) only works with websites and videos, but not files. If you want to work on a file together, you have to ask the other participants to open the file too.