We need to revise what is done to register .jar association in the JRE installer.

From time to time we see reports from people who can not run jar by doubleclick because they happen to have
custom .jar association. There could be multiple scenarios leading to this problem, e.g. user has jar file but no java yet
and tries to open it with some other program, etc. (See for example discussion in RT-21194)

Subsequent JRE installations/uninstallations should correct or offer to correct user settings in HKCU (basically remove .jar association if any).

Note that there is no easy way for user to fix it once it is broken.
User can not use "Open with" menu as this will result in "java a.jar" type of command (and not "java -jar a.jar") that will fail.