/usr/share/kde4/apps/khelpcenter/searchhandlers/docbook.desktop is in khelpcenter4 4:4.8.2-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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DocumentTypes=application/docbook+xml
SearchBinary=htsearch
SearchBinaryPaths=/srv/www/cgi-bin,/usr/lib/cgi-bin,/opt/www/htdig/bin
SearchCommand=/usr/lib/kde4/libexec/khc_htsearch.pl --binary=%b --docbook --indexdir=%d --config=%i --words=%w --method=%o --maxnum=%m --lang=en
IndexCommand=/usr/lib/kde4/libexec/khc_docbookdig.pl --indexdir=%d --docpath=%p --identifier=%i
TryExec=/usr/bin/htdig
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