Switch preamble's "format" option to "mimetype".
Review Request #11699 — Created July 1, 2021 and submitted
To help stay future-proof, the preamble sections'
formatoption is now
mimetype. This supports two options:
This helps the spec stay future-proof, making it easier for tools to
more easily support rendering of other types in the future.
Built the docs. Checked the new sections to ensure the correct wording