The 28-year-old netted 32 goals in 87 appearances for the club after becoming Chris Powell's first signing in management.
Brentford, where he was on loan last season, are reportedly at the head of the queue for the striker's services after his contract expired at The Valley.
"Bradley was my first signing so whatever happens with me and my managerial career, you never forget your first," Powell told the club's official website.
"I remember when I had the opportunity to bring Brad I said 'right, let's do it'. He came in scored goals straight away and fitted in with the group immediately and then scored 22 goals when we got promoted from League One.
"On behalf of myself, all my staff and all my players I want to thank Brad for his contribution. I really wish him well because he's a fantastic young man and a fantastic character and we thank him for his contribution to our wonderful football club."