fix(14): WR-02 derive the screenshot extension from the sniffed image type

The public screenshot kept the client file's extension, so a valid image
named x.html or x.svg was stored and served under that type. The disk
name now carries the client extension only when it names the sniffed
type, else the type's own; file_name keeps only the client's base name.

feedback_test.go

@@ -617,6 +617,30 @@ func TestFeedbackSubmit(t *testing.T) {
 		}
 	})
 
+	t.Run("client-extension-never-names-the-public-file", func(t *testing.T) {
+		_, app, gdb := newHandler(t)
+		bucket, err := blob.OpenBucket(t.Context(), "mem://")
+		if err != nil {
+			t.Fatal(err)
+		}
+		t.Cleanup(func() { _ = bucket.Close() })
+		for name, want := range map[string]string{
+			"x.html": ".png", "../../x.svg": ".png", "noext": ".png", "Shot.PNG": ".png",
+		} {
+			sub := &models.Submission{Message: "x", Type: "bug", PageURL: "https://widget.example.test/"}
+			if err := classes.StoreSubmission(t.Context(), app, gdb, bucket, sub, &classes.Screenshot{Filename: name, Data: pngBytes(t, 0)}); err != nil {
+				t.Fatal(err)
+			}
+			file, err := classes.ScreenshotOf(t.Context(), gdb, sub.ID)
+			if err != nil || file == nil {
+				t.Fatalf("screenshot %v %v", file, err)
+			}
+			if !strings.HasSuffix(file.DiskName, want) || strings.Contains(file.FileName, "/") {
+				t.Fatalf("%q stored as %q (file_name %q)", name, file.DiskName, file.FileName)
+			}
+		}
+	})
+
 	t.Run("rolled-back-write-queues-nothing", func(t *testing.T) {
 		_, app, gdb := newHandler(t)
 		subsBefore := countRows(t, gdb, `SELECT COUNT(*) FROM golem15_feedback_submissions`)