いつも忘れるので
this.getRequestCycle().setRequestTarget(new ResourceStreamRequestTarget(new AbstractResourceStreamWriter() { private static final long serialVersionUID = 1L; public String getContentType() { return "application/octet-stream;charset=MS932"; } public void write(OutputStream output) { //ダウンロード処理 } }, "download.csv"));
みたいな感じ