Contains methods for bulk importing attachment data into the attachment storage subsystem. Importantly, it assumes
that the Attachment metadata already exists in the database.
(ReadOnlyFileBasedAttachmentStore source, int concurrency, Option<Effect<Attachment>> onCompleteAttachment)
Import attachment data from the specified file system source with specified concurrency, and an optional effect
that is run after each attachment is imported (e.g. to update a task progress sink). The import operation is
terminated if there is an error.
||The file system source of attachment data
||The number of concurrent attachment imports
||Optional effect that is run after each attachment is imported.