I have the same issues with my files I downloaded from https://www.cgtrader.com. I am not able to find the exact link but here are some code-snippets (I hope they are helpful). Here are the first lines of my obj.file which has more than 19.000 lines and 13MB (it's a tree).
...
and here is my mtl-file:

newmtl ROOTS2
Ka 0.1 0.1 0.1
Kd 1 1 1
Ks 0 0 0
illum 2
Ns 2
map_Kd OC12brk2.tif
map_bump
...

I also have several tif-files. Both mtl- and obj-files are in the same zip-folder when importing, but colors are not imported.
Please help me
Thanks in advance.