qq.FileUploader: Cancel submit

I'm using the plugin qq.FileUploader.

Before submitting file, I want to know if a file with the same name has already been upload.

I'm using this code:

var uploader = new qq.FileUploader({
        element: document.getElementById('file-uploader-requestDocuments'),
        action: '<%: Url.Action("Create", "RequestDocument") %>',
        params: { id: $('#RequestTempUploadFolderID').val() },
        sizeLimit: 10520000,
        onSubmit: function (id, fileName)
            $('#file-uploader-requestDocuments').find('.qq-upload-file').each(function () {
                if ($(this).text() == fileName) {
                    return false;
                return true;
            });
        }
    });

The return false is correct, but it does not stop the submit!

How stop the file upload, and there is a better way to get the file already uploaded?

how about doing that checking in your server side?! and returning a response in AJAX, for duplicate file names?!

EDIT

if your server response this:

{"success":"false", "errorMessage":"File name is duplicate!"}

you can have that JS code:

var uploader = new qq.FileUploader({
        element: document.getElementById('file-uploader-requestDocuments'),
        action: '<%: Url.Action("Create", "RequestDocument") %>',
        params: { id: $('#RequestTempUploadFolderID').val() },
        sizeLimit: 10520000,
        onComplete: function(id, fileName, responseJSON){
            if(!responseJSON.success){alert(responseJSON.errorMessage);}
        }
    });