title: Use Zend 2 Extension Validator only if file is uploaded
body: <p>I would like to validate the file extension only if a file is uploaded.</p>
  
  <p>I have a collection of fieldsets that include a File Input element. If I want to upload a file for only one of the fieldsets and leave the rest of the File Input elements empty the form is not validated, although they have required = false. This validation triggers the "fileExtensionNotFound" error.</p>
  
  <p>Is there a way to add the AllowEmpty to or before the Extension validator?</p>
  
  <pre><code>        'file' =&gt; array(
              'required' =&gt; false,
              'validators' =&gt; array(
                  array(
                      'name' =&gt; 'Zend\Validator\File\Extension',
                      'options' =&gt; array(
                          'extension' =&gt; array('pdf', 'xls','doc'),
                      )
                  )
              )
          )
  </code></pre>