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As the title says, how would you go about doing this. I have looked up solutions but none work. They may have worked in older versions of Unity but not in 2020.1
If load the OBJ in a software such as Blender with the .mlt file next to it, I believe it should automatically apply the texture. Then from there, you can export the file from blender. What are you trying to accomplish? How did you get a .mlt file?