custom texture armour

I make an armour, then I get the armour texture file and use Mcreator's internal texture editor, and then it turns to a .PNG file instead of an armour texture file. Is there a way to edit an armour texture file without turning it into a .PNG file? I also tried editing the armour texture file in Blockbench but it still can't export the texture as an armour texture file. I have seen lots of very good modders with custom textured/modeled armour and I want to have my own custom textured armour.