How to display back.jpg with Player.Art

So my advancedsettings.xml is:

<advancedsettings version="1.0">
    <musicthumbs>
        <albumextraart><arttype>back</arttype></albumextraart>
    </musicthumbs>    
</advancedsettings>

and i have an album dir with back.jpg

but when I try and display it with Player.Art(back) it doesn't show up.

Same album dir has Thumb.jpg and Player.Art(thumb) or Player.Art(album.thumb) work just fine.

Please someone - what am i doing wrong?!