preinst is in libservice-wrapper-java 3.5.26-3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10  | #!/bin/sh -e
if [ ! -z $2 ]; then
	if dpkg --compare-versions $2 lt 3.5.9-3; then
		test -d /usr/share/doc/libservice-wrapper-java  \
			&& rm -rf /usr/share/doc/libservice-wrapper-java
	fi
fi
 
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