/usr/share/thumbnailers/gnash.thumbnailer is in gnash 0.8.11~git20160608-1.3.
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/gnash-thumbnailer
Exec=/usr/bin/gnash-thumbnailer %u %o %s
MimeType=application/x-shockwave-flash;
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