wordpress mail header set else plain text

Hope to get some help with a piece of code, I am using a theme for wordpress which sets the mail headers to text/html, this causes some problems with plain text mail ex. linebreaks don't show anymore.

I tried setting :

} else {
    return 'text/plain';
}

but I don't know php very well so I don't know where to place it to make it work. I would like to set the text/plain for mails not defined.

this is the code for the wp header :

 /**
 * filter mail headers
 */
function wp_mail($compact) {
    if (isset($_GET['action']) && $_GET['action'] == 'lostpassword') return $compact;
    if ($compact['headers'] == '') {

        //$compact['headers']   = 'MIME-Version: 1.0' . "\r\n";
        $compact['headers'] = 'Content-type: text/html; charset=utf-8' . "\r\n";
        $compact['headers'].= "From: " . get_option('blogname') . " < " . get_option('admin_email') . "> \r\n";
    }

    $compact['message'] = str_ireplace('[site_url]', home_url() , $compact['message']);
    $compact['message'] = str_ireplace('[blogname]', get_bloginfo('name') , $compact['message']);
    $compact['message'] = str_ireplace('[admin_email]', get_option('admin_email') , $compact['message']);

    $compact['message'] = html_entity_decode($compact['message'], ENT_QUOTES, 'UTF-8');
    $compact['subject'] = html_entity_decode($compact['subject'], ENT_QUOTES, 'UTF-8');

    //$compact['message']       =   et_get_mail_header().$compact['message'].et_get_mail_footer();

    return $compact;
}