Bug in YUI. Using Chrome, when using io-upload-iframe to submit a form, hidden inputs added to the form are removed before form submission.
The hidden inputs do need to be removed after submission to prevent these forms from collecting hidden inputs that will lead to incorrect, duplicate, or unnecessary data. Possible solutions: put the cleanup in a timeout or during the complete event.
When using test.html to submit a file upload, debug io-upload-iframe.js. The actual result is the hidden inputs generated from the data object in test.html request are removed before submitting form.