/usr/share/thumbnailers/dia.thumbnailer is in dia-common 0.97.3+git20160930-8.
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=dia
Exec=dia -t png -e %o -s %s %i
MimeType=application/x-dia-diagram;
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