title: Can't get multer filefilter error handling to work
body: <p>I'm playing around with file uploading in Node.js/Multer.</p>
  
  <p>I got the the storage and limits working. But now im playing around with filefilter to simply deny some files by <code>mimetype</code> like this:</p>
  
  <pre><code>fileFilter: function (req, file, cb) {
   if (file.mimetype !== 'image/png') {
    return cb(null, false, new Error('goes wrong on the mimetype'));
   }
   cb(null, true);
  }
  </code></pre>
  
  <p>When a file gets uploaded that is not a PNG it wont accept it. 
  But it also won't trigger the <code>if(err)</code></p>
  
  <p>When the file is to big it does generate the error. So somehow I need to generate an err on <code>filefilter</code> as well but im not sure how and guess <code>new Error</code> is wrong.</p>
  
  <p>So, how am I supposed to generate an error if the file is not correct? What am I doing wrong?</p>
  
  <p>Full code:</p>
  
  <pre class="lang-js prettyprint-override"><code>var maxSize = 1 * 1000 * 1000;
  
  var storage =   multer.diskStorage({
    destination: function (req, file, callback) {
      callback(null, 'public/upload');
    },
    filename: function (req, file, callback) {
      callback(null, file.originalname);
    }
  });
  
  
  var upload = multer({
     storage : storage,
     limits: { fileSize: maxSize },
     fileFilter: function (req, file, cb) {
       if (file.mimetype !== 'image/png') {
         return cb(null, false, new Error('I don\'t have a clue!'));
       }
       cb(null, true);
     }
  
   }).single('bestand');
  
  
  router.post('/upload',function(req,res){
      upload(req,res,function(err) {
          if(err) {
                return res.end("some error");
          }
      )}
  )}
  </code></pre>