How to add a new mime type in JBoss EAP 7 / 6

Issue

• We use application test pages with the extension .avail (or .action).
• We need add a new JBoss mime type to our configuration for that extension.
• Is there a way to do this via the CLI or GUI?
• How to serve custom Json error pages from JBoss? Having error pages with .json extension gives the below error :

ERROR [io.undertow.request] (default task-6) UT005023: Exception handling request to /URL : java.lang.IllegalArgumentException: UT000068: Servlet path match failed