mail with attachments is blank in thunderbird
to reproduce:
- send batch of attachments (with swaks: simple body + pdf attachment, 1.1M). 10 mails
- start clicking as a crazy monkey as mails arrive
- some will display body + attachment correclty, other are blank.
In another test, I left the sync to happen w/o clicking on the MUA, and everything was fine.
(from redmine: created on 2015-03-05, closed on 2015-04-30, relates #6754, relates #5010 (closed))