Does the rest of this resource pack work correctly? I downloaded the images you provided, renamed them, and placed them in a resource pack. After reloading the resource pack (F3 + T) they all showed as would be expected.
You saved them in the wrong format. If you check the MIME types of the helmet and chestplate, they're both "WEBP" files, not "PNG" files. Just changing the file extension of a file doesn't change how the game sees the file. For that you'll need to actually have it in the proper format. I'm not sure why they worked for Millstone, but his browser may have converted it without his knowledge. That's just a guess though.
Anyway, I'm not certain that that's your problem but it's definitely a place to start. I hope that helps you.