Operating system: Windows 10
Graphics card: NVIDIA GeForce GTX 950M
Short description of error
When exporting an OBJ of a mesh with a material using an image texture, the MTL file only includes the necessary map_Kd line at the end when using the "Principled BSDF" shader.
If you export an OBJ using another shader, such as "Diffuse BSDF," or no special shader (connecting the "color" of the texture node directly to the "surface" of the output node, when you export an OBJ, the MTL file has no map_Kd line, and the texture is not visible in other applications. You can manually add the map_Kd line, and the model works fine, with the proper texture. But this workaround is not ideal.
Exact steps for others to reproduce the error
If you download the attached blend file, which includes a mesh with a material using an image texture, and export the OBJ, you will reproduce the error. If you go into the "shading" interface and add a "Principled BDSF" shader between the texture and output node, then when you export the OBJ, the MTL will have the map_Kd line and will work properly.