Upload file in formplugin not working

I tried the default upload example in sandbox web and the form did got submitted but did not upload any file.

I tried this as well in my own installation, and could not make it to work. I even copy-pasted the example in FormPlugin#Upload_form and still could not make it to work.

Is this a bug or am I missing something?

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Fixed with version 1.6 of FormPlugin which was released on 26 May 2010; see Tasks.Item8985