I am using this ViewScript for my standard form elements:
element->getId(); ?>\">
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Just in case you have followed @Shaun's answer and you are still getting the error: make sure that you've disabled default decorators for the element in question (take look at line 2):
$this->addElement('file', 'upload_file', array(
'disableLoadDefaultDecorators' => true,
'decorators' => array('File', array('ViewScript', array(
'viewScript' => '_form/file.phtml',
'placement' => false,
))),
'label' => 'Upload',
'required' => false,
'filters' => array(),
'validators' => array(array('Count', false, 1),),
));