I have abandoned my original over-complicted approach to this bug in favour of just saving the attachments to disk and then opening them as regular email files. This solves a lot of problems. I'd rather not have used the disk, but to load things from a string in memory is much more complicated than it looks. Maybe I will come back to that one day.
Bug 1814748 Comment 8 Edit History
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I have abandoned my original over-complicated approach to this bug in favour of just saving the attachments to disk and then opening them as regular email files. This solves a lot of problems. I'd rather not have used the disk, but to load things from a string in memory is much more complicated than it looks. Maybe I will come back to that one day.