v6.4.1: feat(files): Make mime type non-nullable and provide backfill command

Ensures `getMimeType()` always returns a string, eliminating null checks for consumers. The database column is now not nullable, simplifying data consumption.

Introduces a `files:fill-mime-type` console command to backfill missing mime types in existing database rows. This command detects types from file contents, falling back to `application/octet-stream` for missing or unreadable files. It operates on scalar data to prevent `TypeError` during hydration before the column is made non-nullable.