string helper fixed, mime-type reverted

yii/web/Response.php

@@ -10,6 +10,7 @@ namespace yii\web;
 use Yii;
 use yii\helpers\FileHelper;
 use yii\helpers\Html;
+use yii\helpers\StringHelper;
 
 /**
  * @author Qiang Xue <qiang.xue@gmail.com>
@@ -36,12 +37,12 @@ class Response extends \yii\base\Response
 	public function sendFile($fileName, $content, $mimeType = null, $terminate = true)
 	{
 		if ($mimeType === null) {
-			if (($mimeType = CFileHelper::getMimeTypeByExtension($fileName)) === null) {
-				$mimeType='text/plain';
+			if (($mimeType = FileHelper::getMimeTypeByExtension($fileName)) === null) {
+				$mimeType='application/octet-stream';
 			}
 		}
 
-		$fileSize = (function_exists('mb_strlen') ? mb_strlen($content,'8bit') : strlen($content));
+		$fileSize = StringHelper::strlen($content);
 		$contentStart = 0;
 		$contentEnd = $fileSize - 1;
 
@@ -102,7 +103,7 @@ class Response extends \yii\base\Response
 		header('Content-Length: '.$length);
 		header("Content-Disposition: attachment; filename=\"$fileName\"");
 		header('Content-Transfer-Encoding: binary');
-		$content = function_exists('mb_substr') ? mb_substr($content,$contentStart,$length) : substr($content,$contentStart,$length);
+		$content = StringHelper::strlen($content);
 
 		if ($terminate) {
 			// clean up the application first because the file downloading could take long time