I haven't used Owncloud myself, but judging by the docs on ArchWiki, it appears that Owncloud is served by a web server such as Apache or Nginx. If that's the case, then you would just need to mount your media directory to a web-accessible location and then enable automatic directory indexes.
More info:
https://wiki.apache.org/httpd/DirectoryListings#Directory_Li...
http://nginx.org/en/docs/http/ngx_http_autoindex_module.html