/usr/share/postgresql/9.4/extension/pg_prewarm.control is in postgresql-contrib-9.4 9.4.18-0+deb8u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | # pg_prewarm extension
comment = 'prewarm relation data'
default_version = '1.0'
module_pathname = '$libdir/pg_prewarm'
relocatable = true
|