in our 1.5 jnlp specification, it says both extensions and mime-type attribute are required when using the association element:

http://jcp.org/aboutJava/communityprocess/maintenance/jsr056/jnlp_spec.html#4756982

however in 1.5 javawebstart, we allow user to only specify either file extension or mime-type, which is a violation of the specification.