"is Undefined" Error When Calling Function

I'm using a JavaScript to upload multiple files, namely this one: [URL]

The script has the ability to post additional parameters to the server, by calling the function setParams

var doktyp = "default";
var uploader = new qq.FileUploader({
element: document.getElementById('file-uploader-scope'),
action: 'uploads.php',

[Code]....

Without the onSubmit: part the script works well and does as it's supposed to, but with it FireBug reports "uploader is undefined" und the script seizes to function.
Now JavaScript istn't my strong suit, so I have been reading up on this for the last two hours, tried calling that function from a different place, tried to modify it, but no luck; the error remains the same (or slightly different; if I point at the class in question directly, the function doesnt work...).