Support saving variable length rational TIFF tags by default

Tests/test_file_libtiff.py

@@ -367,6 +367,15 @@ def test_whitepoint_tag(
             assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
             assert reloaded.tag_v2[318] == pytest.approx((0.3127, 0.3289))
 
+        # Save tag by default
+        out = tmp_path / "temp2.tif"
+        with Image.open("Tests/images/rdf.tif") as im:
+            im.save(out)
+
+        with Image.open(out) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
+            assert reloaded.tag_v2[318] == pytest.approx((0.3127, 0.3289999))
+
     def test_xmlpacket_tag(
         self, monkeypatch: pytest.MonkeyPatch, tmp_path: Path
     ) -> None:

src/PIL/TiffImagePlugin.py

@@ -252,6 +252,7 @@
     (II, 3, (1,), 1, (8,), ()): ("P", "P"),
     (MM, 3, (1,), 1, (8,), ()): ("P", "P"),
     (II, 3, (1,), 1, (8, 8), (0,)): ("P", "PX"),
+    (MM, 3, (1,), 1, (8, 8), (0,)): ("P", "PX"),
     (II, 3, (1,), 1, (8, 8), (2,)): ("PA", "PA"),
     (MM, 3, (1,), 1, (8, 8), (2,)): ("PA", "PA"),
     (II, 3, (1,), 2, (8,), ()): ("P", "P;R"),
@@ -1177,6 +1178,7 @@ def _open(self) -> None:
         """Open the first image in a TIFF file"""
 
         # Header
+        assert self.fp is not None
         ifh = self.fp.read(8)
         if ifh[2] == 43:
             ifh += self.fp.read(8)
@@ -1343,6 +1345,7 @@ def _load_libtiff(self) -> Image.core.PixelAccess | None:
         # To be nice on memory footprint, if there's a
         # file descriptor, use that instead of reading
         # into a string in python.
+        assert self.fp is not None
         try:
             fp = hasattr(self.fp, "fileno") and self.fp.fileno()
             # flush the file descriptor, prevents error on pypy 2.4+
@@ -1936,9 +1939,10 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
                     types[tag] = TiffTags.LONG8
                 elif tag in ifd.tagtype:
                     types[tag] = ifd.tagtype[tag]
-                elif not (isinstance(value, (int, float, str, bytes))):
-                    continue
-                else:
+                elif isinstance(value, (int, float, str, bytes)) or (
+                    isinstance(value, tuple)
+                    and all(isinstance(v, (int, float, IFDRational)) for v in value)
+                ):
                     type = TiffTags.lookup(tag).type
                     if type:
                         types[tag] = type