formdata files array not working in firefox, but works in chrome and opera

I'm using this javascript code, per Mozilla docs, but it's not working in Mozilla, just in Chrome and Opera (don't know if it's working in ie 10--FormData is supposedly supported in IE version 10--but I'm only really concerned about Firefox).

Code:

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var data = new FormData(document.getElementById("uploadform"));               
var selected_file = document.getElementById('file').files[0];       
data.append('file-0', selected_file);

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firefox also causes server error on the ajax file upload:

Code:

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[Wed Jun 05 21:34:53 2013] [error] [client ::1] PHP Notice:  Undefined index: file-0 in /home/jeff/public/website.com/public/upload.php on line 11, referer: http://localhost/

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I have no idea why "files" does not seem to be working. I've posted on Mozilla support forum with no answers, and on stack overflow with one attempted answer. Here's the relevant Mozilla docs page: Using FormData Objects.

I've tested firefox 21, 24 nightly, and 10 on a linux machine, and tested firefox 21 on a windows machine, all with the same results.