The File element requires it’s own decorator – Zend_Form_Decorator_File.
$this->setElementDecorators(array(
'File',
'Errors',
array(array('data' => 'HtmlTag'), array('tag' => 'td')),
array('Label', array('tag' => 'th')),
array(array('row' => 'HtmlTag'), array('tag' => 'tr'))
));
[edit]
Have just noticed that you are also using other form elements.
After your original code, add:
$this->getElement('Excel')->setDecorators(
array(
'File',
'Errors',
array(array('data' => 'HtmlTag'), array('tag' => 'td')),
array('Label', array('tag' => 'th')),
array(array('row' => 'HtmlTag'), array('tag' => 'tr'))
)
);
That way, ViewHelper is added to all other elements, and for your File element File is used instead.