Fix up formatting of messages with attachments
Fix up formatting of messages with attachments

Previously, all messages would have "---" as the start, even if there
was no leading text.  Make it so the dashes are only added if necessary.

megalithic/wee-slack,wee-slack/wee-slack,scharfie/wee-slack,rawdigits/wee-slack,trygveaa/wee-slack