As it says in the comments of that file:

 /// <summary>
    /// This Handler enables you to take control of screen navigation once a page is saved.
    /// To do this, create  your own page in your own Visual Studio project
    /// and transfer into the admin folder via a post build event. 
    /// Leaving the code as it is, the navigation will just redirect back to the PageSettings page, but you could
    /// actually direct the user to anywhere you like.
    /// </summary>    

You should not modify mojoportal code, but in special cases you can replace an existing file with a custom file. But once you do that it is up to you to maintain the file you replaced during upgrades and understand any implications of doing it.

So in this case you could replace the .ashx file with a custom one that inherits from your own dll or using inline code. Doing that kind of thing is up to you, it is not something I personally recommend doing. This .ashx file was added at the request of someone in the community to solve something he needed to do, it was not my idea. Replace files at your own risk based on your own knowledge and decisions. Doing things like that is right on the edge of forking the code but not quite as bad as truly forking the code when you keep it limited to something you can maintain while still upgrading to new versions of mojoPortal.