title: TinyMCE image uploading overriding images_upload_handler
body: <p>I've got TinyMCE and drag-and-drop image uploading working well. I've specified <code>images_upload_url: 'handler.php'</code> and have written the handler which receives the data and handles it appropriately.</p>
  
  <p>However there is one single extra parameter I would like to dynamically pass up to the handler.php from the client side at the time an image is dropped onto the page. </p>
  
  <p>I see in the documentation that you can "roll your own" function by writing your own images_upload_handler function. That would suffice, however I'd rather use the existing TinyMCE one for simplicity. I was wondering is there a way I can somehow dynamically change the images_upload_url to dynamically append a GET parameter (eg ?foo=bar) just before the default images_upload_handler function runs? Or is there a way I could specify my own function for images_upload_handler, but then call the default 'parent' TinyMCE function straight after? I'm thinking something like this (just as a mockup/idea):</p>
  
  <pre><code>tinyMCE.init({
      selector: 'textarea',
      images_upload_url: 'handler.php',
      images_upload_handler: function (blobInfo, success, failure) {
          // change images_upload_url to handler.php?foo=bar
          // call default TinyMCE images_upload_handler function. 
      }
  })
  </code></pre>
  
  <p>Or alternatively, rather than overriding the images_upload_handler directly, is there a function or event I can hook into after the image is dropped onto the editor, but before the images_upload_handler fires?</p>
  
  <p>Edit:</p>
  
  <p>I tried a different approach of dynamically setting the images_upload_url in the init_instance_callback event as so:</p>
  
  <pre><code>images_upload_url: 'handler.php',
  init_instance_callback : function(ed) {
      ed.settings.images_upload_url += '?foo=bar';
      console.log('changed images_upload_url to: ' + ed.settings.images_upload_url);
  }
  </code></pre>
  
  <p>In that case the console.log actually outputs the changed 'handler.php?foo=bar', however it comes to actually uploading an image, it resorts back to the handler.php - seems it is not changed correctly?</p>