Cannot load more than 2 files to S3 using plupload - amazon-s3

Question

My code below works fine so long as I only load 2 multiple files. If I load 3 or more, every file after the 2nd one does not get added to my s3 bucket (although no error is shown by plupload).
Can anyone understand why?
var uploader = new plupload.Uploader({
browse_button: 'browse', // this can be an id of a DOM element or the DOM element itself
url: 'https://s3.amazonaws.com/mybucket',
filters: {
max_file_size: '25mb',
prevent_duplicates: true,
multiple_queues: true
}
});
uploader.init();
uploader.bind('FilesAdded', function (up, files) {
plupload.each(files, function (file) {
var myFileName = file.name.toLowerCase();
myFileName = myFileName.replace(/[|&;$%#"<>()+,]/g, "");
var elem = $('#hfReplyGuid');
$.ajax({
type: "POST",
contentType: "application/json; charset=utf-8",
dataType: "json",
url: "WebService.asmx/prepareUpload",
data: "{'acl':'private','bucket':'myBucket','file':'" + folderPath + elem.val() + "_" + myFileName + "'}",
success: function (data) {
params[myFileName] = { policy: data.d[0].Policy, signature: data.d[0].Signature };
var multipart_params = {
'key': folderPath + elem.val() + "_" + myFileName,
'filename': folderPath + elem.val() + "_" + myFileName,
'AWSAccessKeyId': AWSAccessKey,
'acl': 'private',
'signature': params[myFileName]["signature"],
'policy': params[myFileName]["policy"],
'success_action_status': '201'
}
up.setOption('multipart_params', multipart_params);
uploader.start();
},
error: function (xhr, ajaxOptions, thrownError) {
alert(xhr.status);
alert(xhr.responseText);
alert(thrownError);
}
});
});
});

Answer 1

I solved this by basing code on this post Upload multiple files to Amazon S3 using plupload
However I had to create the policies in the FileFiltered event, start the uploader in the FilesAdded event and then add each policy in the BeforeUpload event as below:
var multipart_params = policies[file.id];
up.setOption('multipart_params', multipart_params);