Wordpress Function: Loads a single-{cat->slug}.php by category slug or single-{post_type}.php custom template by post type. If these don't exist use single.php.

functions.php

<?php
/**
* single-{category_slug}.php || single-post-type.php || single.php
*
* Gets post cat slug, post type and look matching single- templates.  
* If $categories is not empty, get the category slug, and look for single-{$category_slug}.php. If this file doesn't exist then return single.php.
* If $categories is empty, then check the post_type. If its not equal to post, then we are using a custom post type. Look for
* single-{$post_type}.php.  If this file doesn't exist then use single.php.
* If $categories is empty and post_type is equal to post then use single.php.
*
**/

function add_custom_single_template( $display_custom_single_template ) {

	$object = get_queried_object();
	$post_type = $object->post_type;
	$categories = get_the_category($object->ID);

	if( !empty( $categories ) ) {	
		foreach($categories as $category) {
    			$category_slug = $category->slug;
  		}
		$display_custom_single_template = locate_template("single-{$category_slug}.php");
		if( empty($display_custom_single_template) ) {
			$display_custom_single_template = locate_template("single.php");
		}
		

	} else if( $post_type !== 'post' ) {

		// Locate the post_type single template
		$display_custom_single_template = locate_template("single-{$post_type}.php");
		if( empty($display_custom_single_template) ) {
			$display_custom_single_template = locate_template("single.php");
	}
	
	} else {
		// If no category, then just use the standard single.php template
		$display_custom_single_template = locate_template("single.php");
	}
  	
	if( file_exists( $display_custom_single_template ) )	{
		return $display_custom_single_template;
	}
	
}
add_filter( 'single_template', 'add_custom_single_template', 10, 1 );