OBJ and MTL are companion formats and often used together. The OBJ file defines the geometry, and the MTL file defines the materials that are used. Both OBJ and MTL are text-based formats. A part of an OBJ file looks as follow:
v -0.032442 0.010796 0.025935 v -0.028519 0.013697 0.026201 v -0.029086 0.014533 0.021409 usemtl Material s 1 f 2731 2735 2736 2732 f 2732 2736 3043 3044
The MTL file defines materials as follows:
newmtl Material Ns 56.862745 Ka 0.000000 0.000000 0.000000 Kd 0.360725 0.227524 0.127497 Ks 0.010000 0.010000 0.010000 Ni 1.000000 d 1.000000 illum 2
The OBJ and MTL formats by Three.js are understood well and are also supported by Blender. So, as an alternative, you could choose to export models ...
