Attachments are commonly used for the spreading of viruses. And some of the viruses developed recently spread very rapidly. Orchid has a policy of blocking all types of attachments because of the risk they pose.
Unfortunately, many newer viruses open users mail address books, extract the addresses, and automatically email themselves to some or all of the addresses. The mail will then forge to appear as originating from Orchid.
Remember - Orchid is a text only service!
Do not under any circumstances open any attachments even if they appear to be legitimate Orchid posts.