"Files" property with javascript not working in Firefox, even though it's per Mozilla docs

Question Description

I'm using this javascript code, per Mozilla docs, but it's not working in Mozilla, just in Chrome and Opera.

var data = new FormData(document.getElementById("uploadform"));		
var selected_file = document.getElementById('file').files[0];	
data.append('file-0', selected_file);
firefox also causes server error on the ajax file upload:
[Wed Jun 05 21:34:53 2013] [error] [client ::1] PHP Notice:  Undefined index: file-0 in /home/jeff/public/ on line 11, referer: http://localhost/
I have no idea why the "files" property does not seem to be working on Firefox. Here's the relevant Mozilla docs page: Using FormData Objects.

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Final Answer

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In most cases, the discipline is self-governed by the entities which require the programming, and sometimes very strict environments are defined

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fitting k (56)
Purdue University

Really helpful material, saved me a great deal of time.