The safest way to open attachments is by ensuring that good, up to date AV is in place to scan all email (and attachements before you read them). Even this is no guarantee in the event of a zero-day that the AV vendors haven't caught yet (thankfully they move pretty fast). Also having your system patched with the most current patches helps too.
Most webmail clients these days no have virus scanning built in (such as Yahoo! mail, etc).