Returns a promise for the attachment that you can await. The promise
resolves to either an attachment or null if the attachment has been
deleted meanwhile. The promise is rejected if an error occurs during
the fetch. Note that the AttachmentFetcher itself implementes
PromiseLike, so you await it directly.
Stops fetching the fetcher's associated attachment and cleans up any associated resources.
Note that you are not required to call stop() once your attachment fetch
operation has finished. The method primarily exists to allow you to cancel
an attachment fetch request while it is ongoing if you no longer wish for
the attachment to be made available locally to the device.
These objects are returned by calls to fetchAttachment() on Collection. and allow you to stop an in-flight attachment fetch.