I'm thinking it must be a conflict with another script you have on the page.
The file upload code (located at: ../webassist/kfm/filebrowse.js)
Uses a function called "init()"
one possibility is that there is another function on your page that uses that same name and it is getting overwritten.
You might try a find/replace in that .js file to replace "init(" with "fm_init" to ensure it is unique.
Another likely cause (actually probably more likely) is that the onload event is getting overwritten by other functionality on your page.
The file upload .js file uses this to call the init function onload of the page:
window.onload = init;
It looks like you may have a few other scripts running onload that may overwrite that onload event.
You could try adding the init call to the bottom of your page instead of the onload event, probably right above the </body> tag like:
<script>
init();
</script>
</body>
You may want to try this first and if that doesn't work try renaming the function. One or the other (or both) I imagine will fix it.
