/usr/share/thumbnailers/atril.thumbnailer is in atril-common 1.16.1-2+deb9u1.
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=atril-thumbnailer
Exec=atril-thumbnailer -s %s %u %o
MimeType=application/pdf;application/x-bzpdf;application/x-gzpdf;application/x-xzpdf;application/postscript;application/x-bzpostscript;application/x-gzpostscript;image/x-eps;image/x-bzeps;image/x-gzeps;application/x-dvi;application/x-bzdvi;application/x-gzdvi;image/vnd.djvu;image/vnd.djvu+multipage;image/tiff;application/x-cbr;application/x-cbz;application/x-cb7;application/vnd.comicbook+zip;image/*;application/oxps;application/vnd.ms-xpsdocument;
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