According to this discussion (http://yuilibrary.com/forum/viewtopic.php?f=92&t=6683), YUI Uploader SWF is not returning the response content when the HTTP status code is 4xx or 5xx.
A web service may return different HTTP status code depending on the error that happened, but it could also return a content response, so that the requester may have additional information about the
error.

An HTTP response may look like this:

< HTTP/1.1 100 Continue
< HTTP/1.1 401 Authorization Required
< Date: Tue, 18 Jan 2011 05:51:03 GMT
< P3P: policyref="<hide this>"
< cache-control: public,must-revalidate
< content-length: 68
< content-type: application/json
< Connection: close
{"error":{"xml:lang":"en-US", "detail":"Detailed error message}}

YUI Uploader SWF is only able to detect the HTTP response code (401 in the example above), but it doesn't provide the content that the http has returned.

// subscribing a function to the "uploaderror" event
uploader.on("uploaderror", function(event) {
Y.log(event);
});

Response (removing the other event object properties):
details: {
"0" : {
"id": "file2",
"status": 401,
"type": "uploaderror"
}
}

No response content provided.