This one is a bit of a doozy. A summary of the changes:
- Session has grown storage for attachments which have been uploaded but
not yet sent.
- The list of attachments on a message is refcounted so that we can
clean up the temporary files only after it's done with - i.e. after
copying to Sent and after all of the SMTP attempts are done.
- Abandoned attachments are cleared out on process shutdown.
Future work:
- Add a limit to the maximum number of pending attachments the user can
have in the session.
- Periodically clean out abandoned attachments?
