According to files found by data-miners, the Flamethrower weapon is classed as an LMG. It will also use medium bullets. However, it is assumed that these files will be subject to a lot of changes before being introduced into the Battle Royale mode.
The files that are shown for the ‘Thrower’ indicate that it could either spray fire or lava. One of the codes shows ‘VolcanicPitThrower_Lava’, whilst another states ‘Ranged_Thrower_Flame’.