Edit: removed my solution as it only worked when .gz files were requested.
Courtesy of @PerryAJ :
Add a file called jetty-web.xml to Ignition\webserver\webapps\main\WEB-INF with the following:
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<Configure class="org.eclipse.jetty.webapp.WebAppContext">
<Set name="GzipHandler">
<New class="org.eclipse.jetty.server.handler.gzip.GzipHandler"/>
</Set>
</Configure>
Not sure if this will break anything else 