Rename Object to FileUpload; use native Postgres $n placeholders.

internal/store/postgres_test.go

@@ -2,14 +2,6 @@ package store
 
 import "testing"
 
-func TestRebindPostgresPlaceholders(t *testing.T) {
-	got := rebind(`SELECT a FROM t WHERE x = ? AND y = ?`)
-	want := `SELECT a FROM t WHERE x = $1 AND y = $2`
-	if got != want {
-		t.Fatalf("got %q", got)
-	}
-}
-
 func TestNormalizeUsername(t *testing.T) {
 	if got := NormalizeUsername("  Alice_1 "); got != "alice_1" {
 		t.Fatalf("got %q", got)