/usr/share/mime-info/icedtea-netx-web-start.mime is in icedtea-netx-common 1.5.3-0ubuntu0.14.04.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | application/x-java-jnlp-file
ext: jnlp
|