Missing “Template” dropdown when editing Posts, Pages or Products

If when adding or editing a post (or Page, Product etc) and the dropdown field is missing therefore stopping you from selecting your template file make sure each file includes this line in the header section:

Template Post Type: post, page, product

Here is an example for the full header:

/**
Theme Name:     JG Super Theme
Theme URI:      https://jumpinggiraffe.com
Description:    The best theme on the internet, maybe
Author:         Jumping Giraffe
Author URI:     https://jumpinggiraffe.com
Template:       JG Super Theme
Template Name:	Generic Form
Template Post Type: post, page, product
 */