This file is indexed.

/etc/X11/Xsession.d/00upstart is in upstart 1.12.1-0ubuntu4.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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# Make ubuntu-upstart an alias for ubuntu
if grep -q "^$DESKTOP_SESSION\$" /etc/upstart-xsessions; then
    export XDG_CONFIG_DIRS="/usr/share/upstart/xdg:/etc/xdg"

    BASESESSION=${1% *}
    BASESESSION=${BASESESSION##*/}
    if [ "$BASESESSION" = x-session-manager ]; then
        BASESESSION=$(basename $(readlink /etc/alternatives/x-session-manager))
    fi

    UPSTART=1
fi