wmi accessing indexed properties

I learned a lot today about wmi support in Powershell. Anyway I didn't
manage it to do a simple thing with IIS administration.

I wanted to add a Mime mapping to IisMimeMapSetting.MimeMap property

I did

$wmi = get-WmiObject -namespace root\MicrosoftIISv2 -class
IisMimeMapSetting

But from now I don't know how to continue.

I see the members with

$wmi | get-member

$wmi.MimeMap

results in

POWERSHELL EXCEPTION
EXCEPTION TYPE:System.Management.Automation.CmdletInvocationException
MESSAGE:Exception retrieving members: "Not found "
POSITION:
--------------------------------------------------------------------------------

How can I list the collection? How can I extend the collection?

Tanks, Thomas