/usr/share/postgresql/10/extension/pgmemcache--2.2.0--2.3.0.sql is in postgresql-10-pgmemcache 2.3.0-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | \echo Use "ALTER EXTENSION pgmemcache UPDATE" to load this file. \quit
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