Added additional test

Tests/test_file_tiff.py

@@ -728,6 +728,20 @@ def im_generator(ims: list[Image.Image]) -> Generator[Image.Image, None, None]:
         with Image.open(mp) as reread:
             assert reread.n_frames == 3
 
+    def test_fixoffsets(self) -> None:
+        b = BytesIO(b"II\x2a\x00\x00\x00\x00\x00")
+        with TiffImagePlugin.AppendingTiffWriter(b) as a:
+            b.seek(0)
+            a.fixOffsets(1, isShort=True)
+
+            b.seek(0)
+            a.fixOffsets(1, isLong=True)
+
+            # Neither short nor long
+            b.seek(0)
+            with pytest.raises(RuntimeError):
+                a.fixOffsets(1)
+
     def test_saving_icc_profile(self, tmp_path: Path) -> None:
         # Tests saving TIFF with icc_profile set.
         # At the time of writing this will only work for non-compressed tiffs