title: Dropzone.js rejected files in file previews
body: <p>I have a simple dropzone instance for document upload (only specific file extensions). When I drop a file with other than desired extension on the dropzone element, it shows a message about wrong file extension (which is ok) and creates the same element it would create for a valid file. </p>
  
  <p>Is there some option to show only the message and prevent creation (or showing) of the rest of the element? Or an event fired on file rejection? I noticed that the element of rejected file has class 'dz-error', so I suppose I could use that somehow in the fileAdded callback, but I also think there must be a better way. JS code for reference below.</p>
  
  <pre><code>var accept = ".pdf,.doc,.docx,.odt";
  
  $(document).ready(function(){
      var DZ = $(dropzoneSelector).dropzone({
          url: "example/url",
          autoProcessQueue: false,
          uploadMultiple: true,
          clickable: clickableSelector,
          addRemoveLinks: true,
          acceptedFiles: accept,
          createImageThumbnails: false,
          init: function(){
              this.on("addedfile", handleFileAdded);
              this.on("removedfile", handleFileRemoved);
          }
      });
  }); 
  </code></pre>