List articleattachment

Hey all,

I'm trying to display the latest wmv attachment from all
articles on the frontpage. As far as I know the code below
should work. However it ignores the 'length 1' attribute and
it displays all article attachments with the wmv extension.
Am I missing something?? Thanks in advance!!

<!** local>
<!** publication "Het Nieuwsbedrijf">
        <!** issue current>
        <!** section off>
<!** list article order bydate desc>
<!** list length 1 articleAttachment>
<!** if articleAttachment extension is wmv>

...code...

<!** endif>
<!** endlist>
<!** endlist>

<!** endlocal>