When a customized progress jar is defined in an extension with:
<jar href="..." download-"progress"/>
it only works completely if the progress-class attribute is specified in the application-desc, or applet-desc element.

1.) When an applet is run in plugin and the progress-class attribute is specified in a component-desc element, the attribute is not found at all and the default GrayBoxPainter is shown.
2.) when a Java Webstart app is run and the progress-class attribute is specified in a component-desc element, the attribute is found and the custon clas is used the first time the application runs and the content is downloaded, however, sometimes after the application is cached, and it is run again, the DefaultDownloadWindow is shown (because the progress-class attribute is not found).

These problems occur in 6u18 bo3, after the implementation of Custm Progress feature in
RFEs 6853768, and 6875234