Comment 131 for bug 275471

(In reply to comment #124)
> TB never marks attachments with content disposition inline when sending, ...

Default for mail.content_disposition_type is "inline" in TB2 and "attachment" for TB3, thus attachments may be send out with either disposition depending on that setting. Basically, the reverse of bug 452092 would be needed here, having a positive list of MIME types for which inline disposition makes sense to start with and only download those (preemptively or not) if they are inline and such attachments are to be displayed with the message.