In Win 10 (I'm on 1903, but 1809 was the same) you can't seem to choose D8Viewer to be the default in Win 10's "Default apps" config screen. It showed up for the Music one as you can see in the attached screenshot. But for images, video, etc. I can't. I would really like to see that.
Yeah I've been doing that (file type by file type), but it is a pain. I was hoping that in the "Default apps" in Settings it would work, but i guess not. I can choose Winamp, VLC, etc. just not D8Viewer. Sad Panda