MTL file has materials marked as transparent. If you use Blender,
apply Z Transparency to the material and set it's Alpha to 0,
while the texture should have Alpha Calculate set in Image Sampling section
and Alpha to 0 in the Influence section.

MTL file has materials marked as additive. If you use Blender,
set Blend type - Add in the Influence section, for the second texture
related to the material
