What is the correct MIME type for PSD (Photoshop)

I am trying to create an upload area (frontend) for word press users on a site. I’ve got a lot of types opened up through some function calls, but unable to correctly get PSD support. I’m seeing a lot of different types, and to my knowledge have tried all. I’m not sure if my hosting environment makes any difference. Am on Hostgator VPS.

Anyone successfully gotten WP to upload PSDs?

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;
}