File chooser/picker does not show applicable files

[wezm]

Steps to reproduce:

1. Start Clapper
2. Click Add Files…
3. Navigate to directory with media files

Expected behaviour:

The applicable files are shown.

Actual behaviour:

The COSMIC file chooser shows 'Empty Folder'.

Additional notes:

• Clapper is using a Media files filter specified with MIME types: https://github.com/Rafostar/clapper/blob/8ee82f1fdebac436bde27d3c8e21632c64b1c94c/src/bin/clapper-app/clapper-app-utils.c#L173
• When using the GNOME portal the video file is shown and is selectable
• Clapper uses a video/* mime filter. I've not been able to work out where this is actually matched against files
• xdg-desktop-portal-cosmic version: 1.0.0 (Arch Linux)