/usr/share/thumbnailers/gnome-font-viewer.thumbnailer is in gnome-font-viewer 3.22.0-1+b1.
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=gnome-thumbnail-font
Exec=gnome-thumbnail-font --size %s %u %o
MimeType=application/x-font-ttf;application/x-font-pcf;application/x-font-type1;application/x-font-otf;
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