Custom video covers aren't showing as thumbnails

[Alexandra0056]

When adding custom image as video cover (in most cases using app called Mp3Tag) the custom image does not show as thumbnail in XnView MP Browser, since the app generates thumbnail from frames of video itself, despite manually-added video cover should override the automatically-generated thumbnail (for example Windows Explorer is doing it correctly).

I tried adding .png covers on .mp4 video files.

With term "custom video cover" I meant equivalent to music album covers.

Steps to reproduce:
1. Download & Install Mp3Tag [https://www.mp3tag.de/en/]
2. Open folder with videos in Mp3Tag [for example C:\Users\Public\Videos]
3. Right click on the cover editor and then click on "Add Cover..."
4. Add any image to cover (I tried only with .png)
5. Now open the video in XnView MP Browser and the video is not using custom video cover.

To understand step 3. this is cover editor:

[user0]

Custom thumbnails for videos not displaying

[Alexandra0056]

Custom thumbnails for videos not displaying

Yes, it is technically duplicate. However that post is in "General Support" subforum, so it isn't bug report, while this post is in "Bug reports" subforum.