title: Prepend/Append content to a MVC FileStreamResult
body: <p>I'm re-factoring some code which uses an handler (ashx) to use an MVC <code>FileStreamResult</code> instead.</p>
  
  <p>The code returns json, but wraps it in a callback if jsonp is requested.
  (callback is a string some thing similar to "jQuery1112077" etc)</p>
  
  <pre><code>//existing
              context.Response.AddHeader("Content-Type", "text/javascript");
              if (format == "json")
              {
                  context.Response.WriteFile(jsonFilePath);
              }
  
              if (format == "jsonp")
              {
                  context.Response.Write(callback + "(");
                  context.Response.WriteFile(jsonFilePath);
                  context.Response.Write(")");
              }
  
  //MVC
              if (format == "json")
              {
                  Stream stream = new FileStream(jsonDocPath, FileMode.Open, FileAccess.Read);
                  FileStreamResult fs = new FileStreamResult(stream, "text/javascript");
                  return fs;
              }
  </code></pre>
  
  <p>What is the most efficient "MVC" way of prepending and appending to the output stream for my jsonp case.</p>