Error ElggObject::getMimeType()

I've tried to rename File plugin to Image plugin , (I think my user prefer Image plugin than File plugin)

I almost succeded to do it but there is a little problem. There is error:

PHP Fatal error:  Call to undefined method ElggObject::getMimeType() in C:\xampp\htdocs\hahaha\mod\image\thumbnail.php on line 44

I'm confused , why the error says ElggObject::getMimeType() . but getMimeType function is defined in ElggFile , ?

Someone help me please