Add download categories to the class list of the EDD shortcode

gistfile1.php

<?php
function my_child_theme_edd_post_classes() {

	// get the terms from our 'download_category' taxonomy
	$terms = get_the_terms( get_the_ID(), 'download_category' );

	// keep our original edd_download class
	$classes = array( 'edd_download' );

	if ( $terms && ! is_wp_error( $terms ) ) {
		// add each term slug to our classes array
		foreach ( $terms as $term ) {
			$classes[] = $term->slug;
		}
	} 

	// return a string of classes separated by space
	return implode( ' ', $classes );

}
add_filter( 'edd_download_class', 'my_child_theme_edd_post_classes' );