Use association element on java web start

[lucumon]

Hi! I'm willing if you can update your .jnlp file, in order to be able to tell the operating system that .osm and .osm.gz files should be opened with JOSM. This is achieved using the "association" element in the .jnlp file.
Here is an example: ​https://stackoverflow.com/questions/2878914/jnlp-file-association-how-do-i-open-the-file-which-was-double-clicked-on

[taylor.smock]

It looks like this should be fairly easy to do.

native/josm.jnlp

@@ -11 +11 @@
         <icon href="https://josm.openstreetmap.de/logo.png" width="256" height="256"/>
         <icon href="https://josm.openstreetmap.de/logo64.png" width="64" height="64" kind="shortcut"/> 
         <offline-allowed/>
+        <association mime-type="application/geo+json" extensions="geojson"/>
+        <association mime-type="application/gpx+xml" extensions="gpx"/>
+        <association mime-type="application/gzip" extensions="osm.gz"/>
+        <association mime-type="application/vnd.openstreetmap.data+xml" extensions="osm"/>
+        <association mime-type="application/x-bzip2" extensions="osm.bz2"/>
+        <association mime-type="application/x-josm-session+xml" extensions="jos"/>
+        <association mime-type="application/x-josm-session+zip" extensions="joz"/>
+        <association mime-type="application/x-xz" extensions="osm.xz"/>
+        <association mime-type="application/zip" extensions="osm.zip"/>
         <shortcut>
             <desktop/>
             <menu/>

EDIT: It looks like the DTD​ doesn't like association.

[taylor.smock]

This does not appear to work, unfortunately. Taking a look at the IcedTea (webstart) code (​https://github.com/AdoptOpenJDK/IcedTea-Web), while they do parse the entries, they don't actually do anything with them. While it is technically very easy to add the association data, I'm not going to since the syntax for extensions is kind of unclear. Is it just the bare extension (osm), the extension plus the dot (.osm), or a regex (.*.osm). The samples I've seen are more of a regex (*.osm), but without the implementation being there, I don't want to commit something, and have people assume it works when it doesn't.

Note: I don't particularly care about Oracle Webstart (Java 8), as we are planning on moving to Java 11+ (see #17858).