Dumbuya joined the club in the summer of 2013 after leaving League 1 rivals Crawley Town.
The fleet-footed right-back, who is also capable of playing on the wing, is an exciting prospect going forward and tough to beat at the back.
A Sierra Leone international, Dumbuya arrived to a great response from County supporters, having impressed against Notts for both Crawley and Portsmouth during 2012.13.
Despite carrying a groin injury, Dumbuya played through the pain barrier to help the Magpies complete The Great Escape in 2013.14 but underwent surgery in the summer to rectify the issue.
Having worked tirelessly throughout the close-season to get into top shape, the defender is looking to enjoy an uninterrupted campaign this time around.