Minim not understanding m3u files

[windbag]

Just installed Minim on a new QNAP containing my music in a directory tree:
Typical structure:
artist group/artist/music files/more tree/*.mp3
artist group/artist/playlists/more tree/*.m3u

The m3u playlists are all relative so the whole "artist" chunk can be relocatable.

Using Bubble uPnP as the controller.

It sees the directory where the m3u files are, but then seems to interpret each one as another folder. When clicked on directly, it says "This folder is empty"
When the 3 dots to the right brings up the sub-menu and Play or Play next is selected, it says "No item was found in the direct subfolders of the selected folders"

Why does it think the m3u files are folders ?

For info, I have a copy of the music on my Android (Samsung) phone.

Bubble sees this OK and plays the playlists OK, which seems to point to a server (ie Minim) issue.

Anyone got ideas ?