* Accept caller-supplied attachment dimensions
Forward positional WIDTHxHEIGHT metadata supplied by the caller. Validate dimensions before sending and retain automatic image detection for omitted entries.
* Accept caller-supplied attachment BlurHashes
Forward positional BlurHashes supplied by the caller alongside attachment dimensions. Reject BlurHashes whose length doesn't match their size digit.
* Skip image probing when dimensions are supplied
Caller-supplied dimensions replace the probed values, so don't buffer and decode the image to measure it.
