attachment-reminder.user.js

// ==UserScript==
// @name           Attachment forgetter preventer
// @namespace      attachment-forgetter-preventer
// @description    Prevent forgetting attachments
// @match          http://code.google.com/p/chromium/issues/*
// @version        0.2
// ==/UserScript==

(function(document) {
  document.addEventListener('submit', function(e) {
    var comment = document.querySelector('textarea[name="comment"]');
    if (comment && /attach/i.test(comment.value)) {
      var allBlank = Array.prototype.every.call(
          document.querySelectorAll('input[type="file"]'),
          function(el) { return !el.value; });
      if (allBlank && !confirm('It looks like you meant to attach something. Continue submitting?'))
        e.preventDefault();
    }
  });
}(unsafeWindow.document));

<script src="https://gist.github.com/danbeam/55c45021e1ce268edfaf.js"></script>