Ignore EXTRASAMPLES tag from separate planes image when saving

Tests/test_file_libtiff.py

@@ -1055,10 +1055,15 @@ class TestFileLibTiff(LibTiffTestCase):
         with Image.open("Tests/images/tiff_strip_planar_16bit_RGBa.tiff") as im:
             assert_image_equal_tofile(im, "Tests/images/tiff_16bit_RGBa_target.png")
 
-    def test_separate_planar_extra_samples(self) -> None:
+    def test_separate_planar_extra_samples(self, tmp_path: Path) -> None:
+        out = tmp_path / "temp.tif"
         with Image.open("Tests/images/separate_planar_extra_samples.tiff") as im:
             assert im.mode == "L"
 
+            im.save(out)
+        with Image.open(out) as reloaded:
+            assert reloaded.mode == "L"
+
     @pytest.mark.parametrize("compression", (None, "jpeg"))
     def test_block_tile_tags(self, compression: str | None, tmp_path: Path) -> None:
         im = hopper()

src/PIL/TiffImagePlugin.py

@@ -1768,6 +1768,12 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
         legacy_ifd = im.tag.to_v2()
 
     supplied_tags = {**legacy_ifd, **getattr(im, "tag_v2", {})}
+    if supplied_tags.get(PLANAR_CONFIGURATION) == 2 and EXTRASAMPLES in supplied_tags:
+        # If the image used separate component planes,
+        # then EXTRASAMPLES should be ignored when saving contiguously
+        if SAMPLESPERPIXEL in supplied_tags:
+            supplied_tags[SAMPLESPERPIXEL] -= len(supplied_tags[EXTRASAMPLES])
+        del supplied_tags[EXTRASAMPLES]
     for tag in (
         # IFD offset that may not be correct in the saved image
         EXIFIFD,