title: Dynamic file name with custom outputter
body: <p>I'm trying to process images (create thumbnail images) using u-sql with custom outputter and trying to output files with dynamic file name.</p>
  
  <p>My u-sql code look like this.</p>
  
  <pre><code>REFERENCE ASSEMBLY [USQLAssemblies];
  
  @image_out =
  SELECT USQLAssemblies.ImageOps.scaleImageTo(ImgData, 480, 480) AS thumbnail_image,
         FileName + "480" AS FileName
  FROM dbo.ThumbnailImages;
  
  
  OUTPUT @image_out
  TO @"D:\Test\{FileName}.gif"
  USING new USQLAssemblies.ImageOutputter();
  </code></pre>
  
  <p>The script returned an error.</p>
  
  <blockquote>
    <p>Error: Data partitioned output is not supported for user-defined outputters.</p>
  </blockquote>
  
  <p>Does u-sql support custom outputter with dynamic file name? or is it in preview?
  Any suggestion for workaround?</p>