v6.5.1: feat(files): Improve mime type backfill to re-evaluate default values

The `files:fill-mime-type` command now treats `Helpers::DEFAULT_MIME_TYPE` as a sentinel indicating an unknown type, rather than a detected one. This allows the command to effectively run after a migration has made the mime type column non-nullable and populated empty values with the default.

Additionally, it now skips rewriting values that would resolve to the same mime type, making repeated runs idempotent and efficient.