Strictly speaking the <video/>-references are invalid as the referenced
file must be MKV or WebM, but neither Flathub nor `appstream-util`
have complained about it.

Some hints were provided by ovari:
https://git.jami.net/savoirfairelinux/ring-client-gnome/issues/1117#note_16987

Change-Id: I4db1c85d92c1d0d1f75eb571c3a00358c44d0def