And just to add to the fun....
There is no mention in an e-mailed forum message that there is an attachment. In fact, I'm pretty sure the instant notifications go into the queue before the attachment has even been chosen. So it's perfectly ok for the URL to the attachment to be relative to its community, since it's never used outside of the community.
Enter bulk mail, which has the opposite requirement. I think I will have to add a full_url field to attachments, or some such thing, because the URL to the attachment has to be usable from the e-mail message that's sent out.
And, last but not least, when you mount attachments you have to give it the folder_id of the file-storage folder to use for storing the attachments. I'm trying to make this not depend on dotLRN, since bulk-mail and attachments can both theoretically be used without it, so my only obvious choice for the folder to use is the topmost root folder. Ick.
Suggestions on better approaches welcome - I was hoping to make this something I could contribute to the toolkit, although as the kludges pile up I'm beginning to have doubts.