If AssetStudio isn't saving obj with a .mlt you will need to may need to look at doing things differently. The mlt files hold the material settings and textures data. With out it all you got is gray model.
A: See if AssetStudio can save the model in another format, Models-resource only takes .dae and .obj with matching .mlt model files so you will have to use Blender to import any other format that AssetStudio may save and save the model as one of the formats that Models-resource takes.
B: Edit the obj in Blender and put the textures back onto the model and save as an .obj.
A: See if AssetStudio can save the model in another format, Models-resource only takes .dae and .obj with matching .mlt model files so you will have to use Blender to import any other format that AssetStudio may save and save the model as one of the formats that Models-resource takes.
B: Edit the obj in Blender and put the textures back onto the model and save as an .obj.