title: Zend File Validator return wrong message
body: <p>below code return "File '' is not readable or does not exist" always:</p>
  
  <pre><code>$filters = array(
      '*' =&gt; 'stringTrim'
  );
  $validators = array(
      'image'=&gt; array(
          'allowEmpty' =&gt; TRUE,
          new Zend_Validate_File_ImageSize(array('minheight'=&gt;0,'minwidth'=&gt;0,'maxheight'=&gt;1024,'maxwidth'=&gt;1024)),
      )
  );
  $input = new Zend_Filter_Input($filters, $validators);
  $input-&gt;setData(array_merge($data, $_FILES));
  if (!$input-&gt;isValid()) {
      $this-&gt;_errors = $input-&gt;getMessages();
  }
  </code></pre>