Hey there,
Maybe a moot point now, but if you look at your second screenshot showing the Library panel, you'll see that your mp4 & webm files are grouped together. This is default behavior when you import video files that have the same name but different file formats. If your files don't have the same name, you can still group them manually by dragging/dropping the differently-named video file into the desired video group in the Library.
hth,
Joe
