V174: repair misclassified media rows (file_type / mime_type).

The corruption

Two writer defects, both since fixed at the write boundary (Storage.store_file_in_buckets/7), left rows whose stored classification contradicts what the file demonstrably is:

• file_type poisoned by the caller. The storage writer stored whatever file_type the call site claimed, unchecked. A live example: an external module's upload path hardcoded "image" while its picker accepted audio and video too — every .mov and .mp3 dropped on a board landed as file_type: "image", and the media page trusts that column everywhere (broken <img> tiles, invisible to the video/audio filters, image variant processing run against a QuickTime file).

• mime_type guessed from a map with no audio entries. The writer discarded the mime the caller observed and re-derived it from the extension via a hand-rolled map that knew no audio type at all, so every mp3/m4a/wav was stored — and served (file_controller answers with the instance's mime) — as application/octet-stream.

The repair

Same evidence-over-claim rule the write boundary now applies, in SQL:

1. Rows (files AND file instances) whose mime is blank/octet-stream but whose extension is in the known audio set get the real audio mime — mirrors Storage's @audio_mime_fallbacks.
2. Files whose (now-repaired) mime contradicts a generic file_type take the mime's classification — mirrors Storage.determine_file_type/2's mime branch. Rows with no evidence (unknown mime) keep their claim, and non-generic system types ("tile") are never touched: internal machinery chose those deliberately and the classifier knows nothing about them.

Width/height/duration and missing variants are NOT backfilled here — the variant pipeline owns those, and re-queueing jobs is not a migration's business. A repaired row simply renders correctly from now on.

Down

Deliberately a no-op (marker stamp only). The pre-repair values were wrong, there is no record of which rows held them, and re-corrupting data is not a rollback anyone wants.