Unable to upload any number of files after (uploading a single, cancelling it) x 2

And along with setting "appendNewFiles: false" on Uploader instantiation, this will take care of the bug in the multi-files uploader demo:

BUG 1237
	status: open

	problem:
		unable to upload any number of files

	reproduce:
		1. add a single file
		2. upload it
		3. cancel it
		4. repeat steps 1-3 once
		5. add another single file
		6. click upload
/yui3/ticket/2532312
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