onDeterminingFilename property

During the filename determination process, extensions will be given the opportunity to override the target DownloadItem.filename. Each extension may not register more than one listener for this event. Each listener must call suggest exactly once, either synchronously or asynchronously. If the listener calls suggest asynchronously, then it must return true. If the listener neither calls suggest synchronously nor returns true, then suggest will be called automatically. The DownloadItem will not complete until all listeners have called suggest. Listeners may call suggest without any arguments in order to allow the download to use downloadItem.filename for its filename, or pass a suggestion object to suggest in order to override the target filename. If more than one extension overrides the filename, then the last extension installed whose listener passes a suggestion object to suggest wins. In order to avoid confusion regarding which extension will win, users should not install extensions that may conflict. If the download is initiated by download and the target filename is known before the MIME type and tentative filename have been determined, pass filename to download instead.