Milsom arrived at Meadow Lane from Rotherham United in June 2015.
The midfielder had helped the Millers to win promotion from League One through the play-offs in 2013-14, coming off the bench in the final at Wembley.
He had previously spent time with Fulham, Bury, Southend and Aberdeen.
His industious and tenacious performances in the middle of the park led to him winning the fan and player votes for the 2016-17 Player of the Season award.
Home kit: Ian Roberts
Away kit: Ian Roberts
Third kit: Available
Boots & socks: Available