Attaching an ics (event) file with text/Calendar mime type - help?

Hi,

I'm tying to attach a .ics file to an admin notification email. I can do this fine in that when the email arrives it has the file attached. However it doesn't behave the same way that an invitation from GCal/Outlook/iCal would, i.e. letting you add the invitation/appointment to your calendar directly from the mail client.

It looks like the reason is down to the 'Content-Type' in the email. When I send one from GCal/Outlook/iCal that works fine, this is what I see in the bottom of the email source:

--[mesage id here]
Content-Type: text/calendar; charset="utf-8"; method=REQUEST
Content-Transfer-Encoding: base64

[encoded string here]

When I attach a file with this code:

add_filter("gform_admin_notification_attachments_4", "add_attachment", 10, 3);
function add_attachment($attachments, $lead, $form){
	$doc = '/invite.ics';
	$upload = wp_upload_dir();
    $upload_path = $upload["basedir"];
    $attachments = array();
    $attachments[] = $upload_path . $doc;
    return $attachments;
}

I get this in the email:

--[mesage id here]
Content-Type: application/octet-stream; name="invite.ics"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="invite.ics"

[encoded string here]

Any ideas how I can enforce the correct content type so the mail clients know how to handle it?

Thanks,
Ben