flatpak-builder/session-helper/xdg-app.gresource.xml

<?xml version='1.0' encoding='UTF-8'?>
<gresources>
  <gresource prefix='/org/freedesktop/XdgApp'>
    <file alias="org.freedesktop.XdgApp.xml">data/org.freedesktop.XdgApp.xml</file>
  </gresource>
</gresources>