Max file size popup warning with Formbuilder

[superstoffe]

Right. I've got this orderform where customers can put orders together with a file (max 20mb). On submit you get redirected to a new page where you get a 'thanks you' message. Thats about it.

Now when I file is bigger than 20mb however, it just redirects to Index instead of warning or going to the thank you page.

So my question is; How do I best set a alert/popup to warn when the file is too big at the same time keeping them at the form without losing what they typed in? Like when you dont fill of the form properly sort of.