Questions about HTML order confirmation email

[haykgrigory]

Hello everyone!

I am building a website based on zen cart, and am now trying to make the order confirmation email look the way I want it to.
I am only able to add or modify text, but HTML code does not show up in the email whatsoever. I have tried editing the following files

Root/email/email_template_checkout.html
Root/includes/classes/order.php
Root/includes/languages/english/checkout_process.php

The dummy account that I created to be able check this feature is subscribed to HTML emails, and I have attached a screenshot of admin page for you guys to confirm that.

My questions are.
1. Why does the customer still get text only confirmation email when they chose html as preferred email format?
2. Which file should I edit for the customer to get the customized order confirmation email?

Any input will be appreciated...