Do not open YCbCr colorspace images as RGB without libtiff

Tests/test_file_tiff.py

@@ -230,6 +230,14 @@ def test_save_rgba(self, tmp_path: Path) -> None:
         outfile = tmp_path / "temp.tif"
         im.save(outfile)
 
+    def test_save_ycbcr(self, tmp_path: Path) -> None:
+        im = hopper("YCbCr")
+        outfile = tmp_path / "temp.tif"
+        im.save(outfile)
+
+        with Image.open(outfile) as reloaded:
+            assert_image_equal(im, reloaded)
+
     def test_save_unsupported_mode(self, tmp_path: Path) -> None:
         im = hopper("HSV")
         outfile = tmp_path / "temp.tif"

src/PIL/TiffImagePlugin.py

@@ -267,10 +267,8 @@
     (MM, 5, (1,), 1, (16, 16, 16, 16), ()): ("CMYK", "CMYK;16B"),
     (II, 6, (1,), 1, (8,), ()): ("L", "L"),
     (MM, 6, (1,), 1, (8,), ()): ("L", "L"),
-    # JPEG compressed images handled by LibTiff and auto-converted to RGBX
-    # Minimal Baseline TIFF requires YCbCr images to have 3 SamplesPerPixel
-    (II, 6, (1,), 1, (8, 8, 8), ()): ("RGB", "RGBX"),
-    (MM, 6, (1,), 1, (8, 8, 8), ()): ("RGB", "RGBX"),
+    (II, 6, (1,), 1, (8, 8, 8), ()): ("YCbCr", "YCbCr"),
+    (MM, 6, (1,), 1, (8, 8, 8), ()): ("YCbCr", "YCbCr"),
     (II, 8, (1,), 1, (8, 8, 8), ()): ("LAB", "LAB"),
     (MM, 8, (1,), 1, (8, 8, 8), ()): ("LAB", "LAB"),
 }
@@ -1588,14 +1586,14 @@ def _setup(self) -> None:
                 # fillorder==2 modes have a corresponding
                 # fillorder=1 mode
                 self._mode, rawmode = OPEN_INFO[key]
-            # YCbCr images with new jpeg compression with pixels in one plane
-            # unpacked straight into RGB values
-            if (
-                photo == 6
-                and self._compression == "jpeg"
-                and self._planar_configuration == 1
-            ):
-                rawmode = "RGB"
+            if photo == 6:
+                self._mode = "RGB"
+                if self._compression in "jpeg" and self._planar_configuration == 1:
+                    # YCbCr images with new jpeg compression with pixels in one plane
+                    # unpacked straight into RGB values
+                    rawmode = "RGB"
+                else:
+                    rawmode = "RGBX"
             # libtiff always returns the bytes in native order.
             # we're expecting image byte order. So, if the rawmode
             # contains I;16, we need to convert from native to image