Its a silly skellington! 

Everything should be fully rigged, each bone should move individually without any stretching, the only exception being the spine-> chest bone, but that is probably easily fixed. Importing the obj file into mixamo WILL cause major amounts of stretching and it...doesn't look good. I figured i'd still upload my obj file though

Exporting it as FBX SHOULD work great with Mixamo. If the skelly decides to be animated backwards, you can just flip the skeleton around and it'll import into it. That should be fixed already, though.

Apply the material001 base color file I included as the skeleton material and the grey/black should appear in the right spot. 

This is my first time using both blender and magicavoxel, so bare with me if its not perfect. I am using it in my project right now, though, and everything is working fine.

CC0 but if you want to credit me, feel free! Also please let me know so i can give it a look!