[Need help] How to disable the submit button if bad file extension?

I have searched but only found how to disable a button, not how to do it if its the wrong extension and I dont know much about JavaScript… I want to disable the submit button of a upload form if the file extension is not gif, jpg, jpeg, png or swf. Similar to [url]http://www.imageshack.us/[/url] but simpler.

The form looks like this:

[code=html]
<form action=”uploaded.php” method=”post” enctype=”multipart/form-data” name=”form1″>
<label for=”ufile”></label>
<div align=”left”>

<input name=”ufile” type=”file” class=”fileField” id=”ufile” size=”40″>
<input type=”submit” class=”Upload” value=”Upload!”>
</div>
<label for=””></label>
<div align=”left”></div>
</form>
[/code]

Thanks ?

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