ZF-5114: Zend_Form_Element_File can not be included as optional fom element
It seems that the Zend_Form_Element_File only works if an file is actually uploaded. If not validation of within Zend_File_Transfer_Adapter_Abstract throws an exception in the ->_getFiles() method, because it could not find any files related to the respective form element. The ->_getFiles() method provides a third argument which could be used to block the exception. This argument however is not used by the calling ->isValid() method.