Validate AsyncFileUpload using java-script. [Resolved]

function UploadStartedPdf(sender, args) {
    var filename = args.get_fileName();
    var fileSize = args.get_length();
    alert(fileSize);
    var ext = filename.substring(filename.lastIndexOf(".") + 1);
    if (ext != 'pdf') {
        throw {
            name: "Invalid File Type",
            level: "Error",
            message: "Invalid File Type (Only .png)",
            htmlMessage: "Invalid File Type (Only .png)"
        }
        return false;
    }
    return true;
}

<ajaxtoolkit:AsyncFileUpload ID="AsyncFileUpload" Width="200" runat="server" OnClientUploadError="uploadError" ClientIDMode="AutoID" OnClientUploadStarted="UploadStartedPdf" OnClientUploadComplete="uploadCompletePdf" CompleteBackColor="White" UploaderStyle="Modern" ErrorBackColor="Red" ThrobberID="Throbber" OnUploadedComplete="AsyncFileUpload_UploadedComplete" />