Upload a film in the node
Go to edit mode of the loop and unfold the node where you want your film. Put the marker where you want the film to end up. Click the plus sign to make it a cross. There you will see an arrow pointing up. Click the arrow and choose a film from your computer. It can take some time to upload the film, but if you press the Save button, you can close the node and continue to work in the loop, while you are waiting.