The former Arsenal youngster, 27, joined the club from Bournemouth in the summer of 2008 and scored 11 goals last season.
"This is a big boost for us," manager Paul Trollope told his club's official website. "I think Jo has really developed over that last year. He is looking looking like a top League One striker."
Meanwhile, the club are carrying out an internal investigation after midfielder David Pipe, 25, was arrested on suspicion of assault after a man suffered a fractured skull when he was hit with a bottle in Bristol.
Pipe has been released on bail following the incident in the early hours of Wednesday morning.