This file is indexed.

/usr/share/squirrelmail/plugins/test/INSTALL is in squirrelmail 2:1.4.23~svn20120406-2ubuntu1.

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
 6
 7
 8
 9
10
11
12
Installing Test
===============

The Test plugin comes with SquirrelMail, so you should
only need to activate it using the configuration utility.

1) Go to your config directory and run conf.pl.  Choose
   option 8 and move the plugin from the "Available Plugins"
   category to the "Installed Plugins" category.  Save and exit.

  $ cd config/
  $ ./conf.pl