It then launches that program to display or 'play' the file. For example, while a browser is downloading and displaying a page, it may come across an MPEG file. MPEG files are one sort of compressed video. Currently no Web browser can turn MPEG files back into video, so they rely on a special helper application called an MPEG viewer to handle this job. So when the browser hits an MPEG file it feeds that file to the MPEG viewer and the viewer plays it back.
Some Web browsers come with some helper applications.
For instance, Netscape Navigator comes with a helper application called Naplayer. Apart from these bundled helper applications, users must configure their browsers to deal with other file types by finding and specifying the helper applications. When using these applications, the whole file must arrive before the helper starts playing it back. Another drawback is that the helper is an independent application that does not coordinate its activities with the Web browser. They don't act as a single application and the user must control both separately.