/etc/X11/Xsession.d/55mate-session_materc is in mate-session-manager 1.12.2-1.
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 13 14 15 16 17 18 19 20 21 22 23  | # If we are running the MATE session, source ~/.materc
BASESTARTUP=${STARTUP%% *}
BASESTARTUP=${BASESTARTUP##*/}
if [ "$BASESTARTUP" = x-session-manager ]; then
    BASESTARTUP=$(basename $(readlink /etc/alternatives/x-session-manager))
fi
case "$BASESTARTUP" in
  mate-session*)
    MATERC=$HOME/.materc
    if [ -r "$MATERC" ]; then
      . "$MATERC"
    fi
    # We prepend /usr/share/mate since its defaults.list actually points
    # to /etc so it is configurable.
    if [ -z "$XDG_DATA_DIRS" ]; then
      XDG_DATA_DIRS=/usr/share/mate:/usr/local/share/:/usr/share/
    else
      XDG_DATA_DIRS=/usr/share/mate:"$XDG_DATA_DIRS"
    fi
    export XDG_DATA_DIRS
    ;;
esac
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