Why can't I upload an OBJ, ZIP, or FBX with the correct textures?

I'm attempting to get a Fortnite character model into Mixamo through Blender. In Blender, the textures look fine, so I export as an OBJ to start. Obviously, OBJ's don't have textures, so how might I be able to add a texture post-mixamo in Blender?

After OBJ's didn't work, I try zipping the given MTL and OBJ files together, and it comes out with a strangely lit model that also doesn't export out of Mixamo with any of the correct tectures. 

Finally, FBX's. These are confusing as all heck. I exported one of my character models as FBX and imported it to Mixamo. It came out with a perfectly textured head, but no textures anywhere else.

Any help is appreciated. I've spent 3 days working something out.