Alexander is currently the club's top scorer with 13 goals, having hit a brace against his former club Leyton Orient in the 2-1 on Saturday, a performance that earned him a place in the League One team of the week.
The striker's agreement to the two-year deal means manager Kenny Jackett has so far retained the services of four players, with fellow striker Neil Harris and defenders Andy Frampton and Scott Barron signing new deals last week.