在Opera浏览器中提交表单时如何删除此对话框窗口？

Using Opera Browser (for 64-bit linux / ubuntu). Why when i click submit it pops up a download option dialog, like shown in the screenshot. I tested it in Chromium/Google Chrome/Firefox/Midori/Arora there it works fine except this Opera? How can i ommit that dialog then?

<form action="/a/userslogin" 
      method="get" 
      id="formusers" 
      name="formuser" 
      target="submitme">
<input type="text" name="username" id="username" value=""/>
<input type="password" name="password" id="password" value=""/>
<iframe id="submitme" name="submitme"></iframe>
<input type="submit" name="button" id="button" value="Submit"/>
</form>

PHP: /a/userslogin

  public function usersloginAction() {
    $return = array(
          'flag' => 'test',
          'result' => 'fail'
    );
    $content = Zend_Json::encode($return);
    $this->getResponse()
            ->setHeader('Content-Type', 'application/json')
            ->setBody($content)
            ->sendResponse();
    exit;
  }