When first using the tool, based on the `--help` output I did not
realize that `raw` was a supported format. Then, upon stumbling on a
GitHub issue that documents this format as being able to stream larger
images directly to disk, I found out that specifying `--format raw`
does not work, and leads to a failure relatively late in the image
upload process.
This documents that, when not specifying `--format`, a default format
of `raw` is assumed.
