Made the required changes 

If you use `IsUrlOk` then it implies that you are returning a `boolean` value. Secondly the name it self should be `IsValidUrl`. In your case you are checking if the file name is provided or not,  if not then read the default document. 
So what you should do is 
* Check if request is for default document or not 
* If yes, then set the filename to default document.