I found a tutorial that showed how to find Minecraft textures and I followed it and opened using 7-Zip. I then opened 1.8.9 .jar file by clicking "Open Archive". After that, I tried opening the .json file but I couldn't. I assume I did something wrong, because the textures should be .png, but didn't work. The tutorial I used was How can I get a hold of Minecraft's default textures?
I don't know why you were trying to open the .json file. Those files are basically pure text and have nothing to do with the textures. Try to open the .jar file using "Open With" and then selecting 7-Zip or WinRAR (I personally use WinRAR).
Once open, the textures should be under assets –> minecraft –> textures