Primeiro, corrija a ordem da parte da regra MATCH'es e depois ACTION
KERNEL=="sd?1", SUBSYSTEMS=="usb", ATTRS{serial}=="222228251A0D", SYMLINK+="mediacenter", ACTION=="add", RUN+="/home/user/mount_filesystem.sh"
para
KERNEL=="sd?1", SUBSYSTEMS=="usb", ATTRS{serial}=="222228251A0D", ACTION=="add", SYMLINK+="mediacenter", RUN+="/home/user/mount_filesystem.sh"