My response would have been just the opposite: Yes, normally a briefcase is a personal item.
The answer you need probably depends on the airline you are flying. American allows purses, briefcases and "other similar items that do not exceed 36 linear inches." I carry a breifcase with a laptop inside as a personal item all the time.
My briefcase goes below the seat in front of me and my carry-on goes in the overhead bin.
-- Suzanne