Check previews and original image downloads separately

tests/test_api.py

@@ -40,7 +40,8 @@ def test_upload_rejects_invalid_data_and_normalizes_valid_images(client):
     assert response.status_code == 200
     session = response.json()
     assert (session["width"], session["height"]) == (128, 80)
-    assert client.get(session["image"]).headers["content-type"] == "image/png"
+    assert client.get(session["image"]).headers["content-type"] == "image/webp"
+    assert client.get(f"/api/sessions/{session['id']}/source.png").headers["content-type"] == "image/png"
     assert client.get(f"/api/sessions/{session['id']}/targets").status_code == 404