Mixamo won't display textures on a model uploaded as zip with obj, mtl and png files

Hi, I'm trying to rig a textured model downloaded from the web. It opens correctly in Blender and 3d Viewer. I followed the advice from earlier discussions here.

My model was already an obj file, I made all the texures into png, then modified the mtl file accordingly (although it still contained folder paths from my PC, maybe that's the issue?), then I've packed everyting into a zip.

But uploading the zip into mixamo.com still gives me a textureless model. What am I doing wrong?
I can't post links to my model here, as it marks my post as spam. Also can't attach a zip 😞