Help uploading a file....

I am rendering my form in module as:

$output .= "<form name='showcase' method='post' enctype='multipart/form-data'>
<input type='text' name='field_promotion_title' id='field_promotion_title' value=\"".$node->field_promotion_title[0]['value']."\" /><br />";

$output .= "<textarea rows=5 cols=50 name='field_promotion_detail' id='field_promotion_detail'>".$node->field_promotion_detail[0]['value']."</textarea><br />";

$output .= "<input type='file' name='showcase_upload'  /><br />";
$output .= "<input type='submit' name='submit_showcase' id='submit-showcase' value='Send' /></form><br />";

after form submit I use:

if( isset($_POST['submit_showcase']) ){
  if($file = file_check_upload("showcase_upload")){
    echo "Yes";
  }else{
    echo "No";
  }
}

But it always return false....
Why.... ?? any clue??