In a previous version of TinyMCE I did not have to specify a callback. I just added an image_upload_handler to create a form and complete the XHR post to my server. Now it seems that the only way to get the file picker icon to show in new version of TinyMCE is to add the callback. But I am trying to use the image_upload_handler instead to handle the file upload on the server side to an S3 bucket.