Source

File: wp-includes/post.php

function wp_count_attachments( $mime_type = '' ) {
	global $wpdb;

	$and   = wp_post_mime_type_where( $mime_type );
	$count = $wpdb->get_results( "SELECT post_mime_type, COUNT( * ) AS num_posts FROM $wpdb->posts WHERE post_type = 'attachment' AND post_status != 'trash' $and GROUP BY post_mime_type", ARRAY_A );

	$counts = array();
	foreach ( (array) $count as $row ) {
		$counts[ $row['post_mime_type'] ] = $row['num_posts'];
	}
	$counts['trash'] = $wpdb->get_var( "SELECT COUNT( * ) FROM $wpdb->posts WHERE post_type = 'attachment' AND post_status = 'trash' $and" );

	/**
	 * Modify returned attachment counts by mime type.
	 *
	 * @since 3.7.0
	 *
	 * @param object $counts    An object containing the attachment counts by
	 *                          mime type.
	 * @param string $mime_type The mime type pattern used to filter the attachments
	 *                          counted.
	 */
	return apply_filters( 'wp_count_attachments', (object) $counts, $mime_type );
}