Function canonicalizeMimeType

Canonicalizes a MIME type for use in encrypted-media-v1 cryptographic operations (key derivation and AEAD AAD).

Sender and receiver MUST apply this identical algorithm (features/encrypted-media.md — Media Type Canonicalization):

1. take the substring before the first ;, dropping any parameters
2. trim leading and trailing ASCII whitespace
3. lowercase using ASCII case folding only
4. reject if the result is empty or does not contain /
5. apply the canonical alias image/jpg → image/jpeg

Adding an alias or normalization step is a breaking media-version change.