Fixed form helper variable error

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
diff --git a/system/helpers/form_helper.php b/system/helpers/form_helper.php
index d6e3e85..692909c 100644
--- a/system/helpers/form_helper.php
+++ b/system/helpers/form_helper.php
@@ -228,7 +228,7 @@
 	 */
 	function form_upload($data = '', $value = '', $extra = '')
 	{
-		$default = array('type' => 'file', 'name' => '');
+		$defaults = array('type' => 'file', 'name' => '');
 		is_array($data) OR $data = array('name' => $data);
 		$data['type'] = 'file';
 		return '<input '._parse_form_attributes($data, $defaults).$extra." />\n";