diff --git a/framework/utils/CFileHelper.php b/framework/utils/CFileHelper.php
index 7250e2c36..0514a34b6 100644
--- a/framework/utils/CFileHelper.php
+++ b/framework/utils/CFileHelper.php
@@ -248,23 +248,18 @@ class CFileHelper
 	 */
 	public static function getMimeTypeByExtension($file,$magicFile=null)
 	{
-		static $extensions, $customExtensions;
+		static $extensions, $customExtensions=array();
 		if($magicFile===null && $extensions===null)
 			$extensions=require(Yii::getPathOfAlias('system.utils.mimeTypes').'.php');
-		else if($magicFile!==null)
-		{
-			if(is_array($magicFile))
-				$customExtensions=$magicFile;
-			else if($customExtensions===null)
-				$customExtensions=require($magicFile);
-		}
+		else if($magicFile!==null && !isset($customExtensions[$magicFile]))
+			$customExtensions[$magicFile]=require($magicFile);
 		if(($ext=pathinfo($file, PATHINFO_EXTENSION))!=='')
 		{
 			$ext=strtolower($ext);
-			if(isset($extensions[$ext]))
+			if($magicFile===null && isset($extensions[$ext]))
 				return $extensions[$ext];
-			else if(isset($customExtensions[$ext]))
-				return $customExtensions[$ext];
+			else if($magicFile!==null && isset($customExtensions[$magicFile][$ext]))
+				return $customExtensions[$magicFile][$ext];
 		}
 		return null;
 	}
diff --git a/tests/framework/utils/CFileHelperTest.php b/tests/framework/utils/CFileHelperTest.php
new file mode 100644
index 000000000..e67dc904c
--- /dev/null
+++ b/tests/framework/utils/CFileHelperTest.php
@@ -0,0 +1,26 @@
+<?php
+
+class CFileHelperTest extends CTestCase
+{
+	public function testGetMimeTypeByExtension()
+	{
+		// run everything ten times in one test action to be sure that caching inside
+		// CFileHelper::getMimeTypeByExtension() is working the right way
+		for($i=0; $i<10; $i++)
+		{
+			$this->assertNull(CFileHelper::getMimeTypeByExtension('test.txa'));
+			$this->assertNull(CFileHelper::getMimeTypeByExtension('test.txb'));
+			$this->assertEquals('text/plain',CFileHelper::getMimeTypeByExtension('test.txt'));
+
+			$path=dirname(__FILE__).DIRECTORY_SEPARATOR.'data'.DIRECTORY_SEPARATOR.'mimeTypes1.php';
+			$this->assertEquals('application/json',CFileHelper::getMimeTypeByExtension('test.txa',$path));
+			$this->assertEquals('another/mime',CFileHelper::getMimeTypeByExtension('test.txb',$path));
+			$this->assertNull(CFileHelper::getMimeTypeByExtension('test.txt',$path));
+
+			$path=dirname(__FILE__).DIRECTORY_SEPARATOR.'data'.DIRECTORY_SEPARATOR.'mimeTypes2.php';
+			$this->assertNull(CFileHelper::getMimeTypeByExtension('test.txa',$path));
+			$this->assertEquals('another/mime2',CFileHelper::getMimeTypeByExtension('test.txb',$path));
+			$this->assertEquals('text/plain',CFileHelper::getMimeTypeByExtension('test.txt',$path));
+		}
+	}
+}
diff --git a/tests/framework/utils/data/mimeTypes1.php b/tests/framework/utils/data/mimeTypes1.php
new file mode 100644
index 000000000..f6752d17a
--- /dev/null
+++ b/tests/framework/utils/data/mimeTypes1.php
@@ -0,0 +1,6 @@
+<?php
+
+return array(
+	'txa'=>'application/json',
+	'txb'=>'another/mime',
+);
diff --git a/tests/framework/utils/data/mimeTypes2.php b/tests/framework/utils/data/mimeTypes2.php
new file mode 100644
index 000000000..2fc0de0ed
--- /dev/null
+++ b/tests/framework/utils/data/mimeTypes2.php
@@ -0,0 +1,6 @@
+<?php
+
+return array(
+	'txt'=>'text/plain',
+	'txb'=>'another/mime2',
+);