start_form 

.

function start_form($multi=false, $dummy=false, $action="", $name="")
{
    // $dummy - leaved for compatibility with 2.0 API
    global $form_nested;

    if (++$form_nested) return;

    if ($name != "")
        $name = "name='$name'";
    if ($action == "")
        $action = $_SERVER['PHP_SELF'];

    if ($multi)
        echo "<form enctype='multipart/form-data' method='post' action='$action' $name>\n";
    else
        echo "<form method='post' action='$action' $name>\n";

}

You need to add an extra parameter and write this function on your specific need. But i think that ajax processing and ordinary form submit can be defined on the submit button function.Try  glance the

 submit

function to know more

I notice that ordinary form and multipart form (for uploading file) works differently. Ordinary form will use normal http request while multipart form use ajax. It is indicated by the url. Ordinary form change the url (if there are get variable, it will be disappear) but in multipart form it doesn't change.

This behavior cause the javascript I made that is placed after submit process is not executed. Is there any way to fix this?

I think best solution is deactivate ajax processing for multipart form.