Why on earth would you like to encode a jar-file into an xml-file?
But anyway, the mime-codes f�r JAR-files are:
application/java
application/java-archive
application/x-java-archive
application/x-jar
You can check the mime-types of a file at
http://mindprod.com/jgloss/mime.html But the best aproach, in my humble opinion, is to have a URL in the XML-file that points to a server from where the jar-files can be downloaded. That is the way it is done in Eclipse Update manager, IntelliJ uppdate manager and other similar applications.