Cannot delete video (mp4, mov etc.) files

[m.Th.]

Steps:

0. (initial setup): Browser on one screen, Full Screen (F11) on the secondary screen. Auto-play for video files = off (I don't think that it matters, but anyway).

1. Go to a folder with an mp4 video file.
2. Press Del and answer 'Yes' at "Are you sure..."

Expected:
The file is deleted.

Actual:
Appears a message from OS (Windows) saying that it cannot delete the file because is in use by XnView MP etc.
After this, the thumb disappears from the thumbs pane but at a refresh (F5) the thumb reappears. Of course the file is not deleted from storage.