the model was in voxel but I painted it in painting per pixel (not in the voxelroom)
in order to paint it easy (with all the colours I want).
I have to go outdoor now and I will answer tomorrow to your next reply.
the problem is that I don't remember all the steps I was following to have this mesh (but I can say I exported it in middle poly in obj for 3dsMAx).
I appreciate your help,
Christophe