javascript onsubmit not working

Question 1

When attaching the event handler to the form element, the scope of the event handler is the form and not the window

<form enctype="multipart/form-data" method="post" onsubmit="return upload(this);">

<script>
    function upload(scope) {
        console.log(scope); // The passed scope from the event handler is
    }                       // the form, and not window
</script>

As input elements inside a form are attached as properties to the form object, where the name is the key, calling upload() in the event handler, where the scope is the form, would equal calling form.upload(), but the form already has an element with that name so form.upload is the upload button, not the upload() function in the global scope.

To solve it, either rename the function or the element

<html>
<head>
    <script>
        function upload(){
                alert("I am an alert box!");
        }
     </script>
</head>
<body>
    <form enctype="multipart/form-data" method="post" onsubmit="return upload();">
    <input type="file" name="file">
    <input type="submit" name="upload2" value="Datei hochladen">
    </form>
</body>
</html>

FIDDLE

Question 2

My problem, I had without knowing a form inside a form I was interacting with the inner one no matter what I do the outer form always executes

Question 3

Add return statement in your code

<script>
function upload(){
    alert("I am an alert box!");
    return false;
}
</script>