Since VLC can open both images and WebM videos you could use VLC.
sudo apt-get update && sudo apt-get install vlc
Caveats:
- From Files, you'll have to select all the files in the current directory, right click on one and click on "Open with VLC";
- To be able to do the above you'l have to setup VLC so that it can open images' MIME types, otherwise "Open with VLC" won't be shown in the context menu if the selection contains both images and WebM videos.
Opening a terminal in Files' current directory and running this:
vlc *
instead works out-of-the-box.
You'll probably want to tick Tools -> Preferences -> All -> Playlist -> Play and Pause so that the images won't be played as a slideshow (this changes the default behavior; playing a specific playlist will override this settings with the playlist's setting):