the code already includes an onChange="put()", to keep a visible text-input updated...
but the problem is: you can tab through the form and highlight the "hidden" File input eliminating the effect. standard solutions for standard text fields don't seem to apply (Firefox).
Firefox is pretty strict with the syntax, are you sure you've got it exactly right?
You should be able to use onBlur="put()" to call put as soon as focus leaves the input field. It's not easy to say more without looking at the page.
field2 is hidden by a div... so the Browse button is all that should be operational. tabindex="-1" is the quick fix in ie- im just wondering if there's a javascript solution for mozilla, et cetera.