Tried once more
( rc3 + 003-xfce-x86_64-2017-12-16 in AF mode)
right-click a video file (webM/mp4) = open with mpv
right-click a audio file (mp3/wav) = open with audacious
Let's create a *.m3u playlist which contents only video files.
Because i am lazy i use the playlist window of audacious.
It doesn't matter if audacious is able to actually play these files (don't try),
one can still put files in this window and export the playlist as:
m3u , pls , xml shareable playlist (xspf) , asxv3 playlist or audpl (audacious playlist)
Let's choose m3u.
Now right-click that file = only audacious and leafpad.
right-click again > open with other application, select mpv media player,click open =
videos are playing.-- I now have the mpv context menu entry like shown above when
right-clicking a m3u file, no matter if they contain audio or video files.
The same work the other way round:
If i have a *.pls file mpv is the default > right-click,other app,audacious >
both are in context menu.
I have no idea why it shouldn't work at your end.