hm, that’s a bug, not a feature: harbour-videoPlayer is able to open it via gstreamer
it’s not a third party plugin. It’s from qt officially and part of https://releases.sailfishos.org/sources/4.0.1.48/sailfish-4.0.1.48-oss.tar.bz2 (Jolla SFOS source: qt5-qtimageformats-5.6.3+git2)