<?php
class PortfolioItem extends Page {
    private static $db = array(
        'TrailerVideoURL' => 'Text',
        'ThumbnailImage' => 'Image'
    );

    static $many_many = array(
        'AlternativeStills' => 'Image'
    );

    function getCMSFields() {

        $fields = parent::getCMSFields();

        $fields->addFieldToTab(
            'Root.Main',
            $uploadField = new UploadField(
                $name = 'AlternativeStills',
                $title = 'Upload one or more images (max 10 in total)'
            )
        );
        $uploadField->setAllowedMaxFileNumber(1);
        $fields->addFieldToTab('Root.Main', new TextField('TrailerVideoURL'), 'Content');

        return $fields;
    }
}
class PortfolioItem_Controller extends Page_Controller {
}

class StillsImageExtension extends DataExtension {
    private static $belongs_many_many = array('AlternativeStills' => 'PortfolioItem');
}

Image::add_extension('StillsImageExtension');