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Re the *.obj file format, yes you are correct. The *.mtl file tells the *.obj file which materials/textures are applied to it. The *.jps should be referenced in the *.mtl file. If they are not listed in that file, they won't be applied.
By the way, you can open and read any *.obj or *.mtl file with a plain text editor. So, using just a text file editor, you can see which *.jpg or other texture files might be being called by the *.mtl file, if you just open and read it.
