i cannot add a texture mapping onto my json file for a custom shield with the vanilla models.

i made the model from blockbench and all seems fine, its just not letting me upload texture mappings. 

here is an example: https://imgur.com/bny3hJ2

it works for my obj files tho, and when i try obj files, it doesnt work and it looks like this ingame: https://imgur.com/a/HLj8YkU

please help me to make the shield, and also find a solution to fix it.