Unable to Validate Required File Upload Correctly

Hi

I've tried adding validation to a file upload box. This works brilliantly on the initial post creation but when I go back into the post to edit the content or title it throws a validation failure as the upload box is not filled out.

Here is the code for my metabox:

public function generateMetaBox($meta_boxes){
        $prefix = 'downloads';

        $prefix=str_replace(' ','_',$prefix);

        // General
        $meta_boxes[]=array(
            'id'=>'general',
            'title'=>'General Download Settings',
            'post_types' => 'downloads',
            'context'    => 'normal',
            'priority'   => 'high',
            'fields'=> array(
                array(
                    'name'              =>  'File',
                    'id'                =>  $prefix.'_file',
                    'type'              =>  'file',
                    'desc'              =>  'Select the file from your computer to upload.',
                    'max_file_uploads'  =>  1,
                    'upload_dir'        =>  WP_CONTENT_DIR.DIRECTORY_SEPARATOR.'uploads'.DIRECTORY_SEPARATOR.'downloads'
                ),
            ),
            'validation' => array(
                'rules'  => array(
                    '_file_downloads_file[]' => array(
                        'required'  => true,
                    ),
                ),
            )
        );

        return $meta_boxes;
    }

Is there a work around for this?

Thanks!