$this->add('File', 'uploadFile', ts('Import Data File'), 'size=30 maxlength=255', TRUE);

public function preProcess()
{
    if (isset($this->_submitFiles['uploadFile'])) {
        $uploadFile = $this->_submitFiles['uploadFile'];
        $importer = new CRM_Importer();
        $importer ->truncate();
        $importer -> importCVStoTable($uploadFile['tmp_name']);
    }
}