map_meta_cap.php

<?php
/**
 * Only allow super admins to upload files
 *
 * @see map_meta_cap()
 * 
 * @param array $caps Primitive capabilities assigned to meta caps.
 * @param string $cap Meta capability.
 * @param int $user_id The current user id.
 * @param array $args An array of contextual arguments, sometimes empty.
 *
 * @return array An array of capabilities.
 */
function wpdocs_map_meta_cap( $caps, $cap, $user_id, $args ) {
	if ( 'upload_files' == $cap && ! is_super_admin( $user_id ) )
		$caps[] = 'do_not_allow';

	return $caps;
}
add_filter( 'map_meta_cap', 'wpdocs_map_meta_cap', 10, 4 );