The File element requires its 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]
Just noticed that you are also using other form elements.
After your source 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')) ) );
Thus, the ViewHelper is added to all other elements, and the File file is used instead.
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