Lets a poster upload a thumbnail that replaces the fetched one. It adds
dumps.custom_thumbnail_mime, which holds the uploaded image's type; NULL
means the dump has none.
f
up(db: DatabaseSync): void
Run once by runMigrations. Existing dumps keep their fetched thumbnails,
and a database made from schema.sql already has the column.
Usage
import * as mod from "api/db/migrations/0002_dump_thumbnail.ts";