/usr/share/thumbnailers/gnome-nds-thumbnailer.thumbnailer is in gnome-nds-thumbnailer 3.0.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | [Thumbnailer Entry]
TryExec=/usr/bin/gnome-nds-thumbnailer
Exec=/usr/bin/gnome-nds-thumbnailer -s %s %u %o
MimeType=application/x-nintendo-ds-rom;
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