flatpak-builder/document-portal/xdg-document-portal.gresource.xml

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