After some time poking around in the CMS, the development site and the production site, and some testing of adding media files and links to them on pages on the site, and regenerating sites, it still appears to me that existing multimedia content on the CanMys site was put there manually, and not through the CMS (as all other pages are). A related issue is how to generate a site from the CMS and then migrate it from the development site to the production site without overwriting (and destroying) any files that were uploaded manually to either the development or production sites. Will try to get hold of AM.