First of all this is a very common problem, for more details see this post http://www.yiiframework.com/forum/index.php/topic/52052-upload-file-with-any-charset/

This is for folks who are using \kartik\widgets\FileInput to upload files from your client. How do you display an image for update after you have uploaded it to the server using the widget?

There are cases that you want to validate the file format, size, extension etc but how to do that without submit the form ?