/usr/share/thumbnailers/geogebra.thumbnailer is in geogebra-gnome 4.0.34.0+dfsg1-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [Thumbnailer Entry]
Exec=/usr/bin/ggthumb %i %o %s
MimeType=application/vnd.geogebra.file;
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