/usr/share/thumbnailers/gnome-web-photo.thumbnailer is in gnome-web-photo 0.10.6-1+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-web-photo
Exec=gnome-web-photo --mode=thumbnail --timeout=16 --thumbnail-size=%s %u %o
MimeType=text/html;application/xhtml+xml;
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