• atom1.atom (application/atom+xml)
• atom1.xml (application/xml)
• rss1.rss (application/rss+xml)
• rss1.xml (application/xml)
• rss2.rss (application/rss+xml)
• rss2.xml (application/xml)
• rss2.goggarss (application/rss+xml)
• atom1.goggaatom (application/atom+xml)

Looks like only RSS v2 with application type 'application/rss+xml' works on my Sony Ericsson W850i.