enforce-https-for-wp-attachment-urls

/**
 * Enforce HTTPS for Attachment URLs
 * @version 1.0
 */
function mycred_pro_enforce_https_for_attachments( $url ) {

    return str_replace( 'http://', 'https://', $url );

}
add_filter( 'wp_get_attachment_url', 'mycred_pro_enforce_https_for_attachments' );