fix: update test fixture files table to use mime_type (no content_hash)

The test fixture's files table was using the old column names (content_type,
content_hash) which don't match the actual generated schema from
storage_module.sql (mime_type, no content_hash on files).

Author: pyramation

graphql/server-test/__fixtures__/seed/simple-seed-storage/schema.sql

@@ -40,8 +40,7 @@ CREATE TABLE IF NOT EXISTS "simple-storage-public".files (
   id uuid PRIMARY KEY DEFAULT uuid_generate_v4(),
   bucket_id uuid NOT NULL REFERENCES "simple-storage-public".buckets(id),
   key text NOT NULL,
-  content_type text NOT NULL,
-  content_hash text,
+  mime_type text NOT NULL,
   size bigint,
   filename text,
   owner_id uuid,