Use a fresh LiveData for each attachment creation task
This branch fixes a bug caused by my suggested changes to !1038 (merged). Since there's a 1:1 correspondence between AttachmentCreationTasks and LiveDatas, I think a better solution would be for the LiveData to be created and returned by the task. But that's a bigger piece of refactoring that may be out of scope for fixing this bug.
Closes #1594 (closed)