No one really knows... There were Li batteries. That's confirmed. Rumours say there were other cargo declared as Mangosteens. But it's off-season for mangosteens. So many say it's pangolin scales, rhino horns, elephant tusks being sent to the Chinese blackmarkets from Malaysia.
Conspiracy theorists claim Malaysia is hiding information. Whatever it is, the Chinese were the first to stop batteries, power banks etc being loaded into luggage after the MH370 disaster. I got caught to this mess in several Chinese airports for domestic flights. Bags everywhere, they were looking for passengers, it was confusion as they just started the practice. Months later all other airlines followed this practice. It seems that the Chinese believe that it was a fire caused by the batteries.
Hopefully one day it will be resolved. There are known facts that cannot be disputed. Inmartsat calculated the position from the satellite signals transmitted by the engines. This should be fairly accurate. But I noticed that they have not made the corrections for temperature inversions in the atmosphere which can cause location errors.