Wordpress - What is the correct MIME type for PSD (Photoshop)

This is what I use to allow PSD upload:

add_filter('upload_mimes', 'custom_upload_mimes');

function custom_upload_mimes ( $existing_mimes = array() ) {
    $existing_mimes['psd'] = 'image/vnd.adobe.photoshop';
    return $existing_mimes;
}

A quite horrible mimetype with several values - all of which are in use:

image/vnd.adobe.photoshop

application/x-photoshop

application/photoshop

application/psd

image/psd