Bug 578654 - Associate "class without source" extension with contenttype

This enforces that the internal "class without source" files are
associated with the JDT Class content-type even if some other
content-types capture them, and then will still allow to open the JDT
Class Viewer for those files.

Change-Id: Ide99663dca2a6f903b79dd65556012ec1328198b
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/190616
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>
Reviewed-by: Sarika Sinha <sarika.sinha@in.ibm.com>

org.eclipse.jdt.ui/plugin.xml

diff --git a/org.eclipse.jdt.ui/plugin.xml b/org.eclipse.jdt.ui/plugin.xml
index e1be8ef..2a297d9 100644
--- a/org.eclipse.jdt.ui/plugin.xml
+++ b/org.eclipse.jdt.ui/plugin.xml
@@ -7799,4 +7799,11 @@
            requiresUIThread="false">
      </javaCompletionProposalComputer>
   </extension>
+  <extension
+        point="org.eclipse.core.contenttype.contentTypes">
+     <file-association
+           content-type="org.eclipse.jdt.core.javaClass"
+           file-extensions="class without source">
+     </file-association>
+  </extension>
 </plugin>