Using ZF 2.1.5, one solution might be setOptions()
.
In the form definiton:
$file = new Element\File('file');
$file->setLabel('Photo (.jpg, .gif, .png)');
$file->setOptions(array('description' => 'test description'));
…
When rendering the form element:
$element = $form->get(…);
var_dump($element->getOptions());
Will give you access to:
array(1) { ["description"]=> string(16) "test description" }