Howdy, everyone! Many years ago, I made a post about missing/messed up textures when importing .obj files from Anim8or into Mixamo, and just last night, I FIGURED IT OUT!! It's a bit strange as to how it works, but bear with me.
You'll need:
-Your model exported as a .obj with the .mtl file that exports with it
-Your textures
Note- You gotta mess around with the specularity of your textures, they can make it look like you have no textures, or that there is a slight shine that is your texture. Try 0.06 for a nice leather shine, 0.01 for a nice rough plasticy shine, 0.1 for hard shine and 1 for blasted lmao
Okay, so, step one:
Zip the .obj and .mtl files into a folder. Name it your model's name.
Step two:
Highlight the textures, right click and press copy.
Step three:
Click into the zipped folder and paste your textures. (its weird, but that's how you've gotta hit this thing.)
Step four:
Open Mixamo, go to upload character, and drag your zipped folder into the part where it says, 'Select character file'
Step five:
IT WORKS!! 
If you have any issues, please comment and I will help you figure it out!
-7c
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