Support writing LONG8 offsets

Tests/test_file_tiff.py

@@ -108,10 +108,6 @@ class TestFileTiff:
             assert_image_equal_tofile(im, "Tests/images/hopper.tif")
 
         with Image.open("Tests/images/hopper_bigtiff.tif") as im:
-            # The data type of this file's StripOffsets tag is LONG8,
-            # which is not yet supported for offset data when saving multiple frames.
-            del im.tag_v2[273]
-
             outfile = str(tmp_path / "temp.tif")
             im.save(outfile, save_all=True, append_images=[im], tiffinfo=im.tag_v2)
 

src/PIL/TiffImagePlugin.py

@@ -2123,7 +2123,7 @@ class AppendingTiffWriter(io.BytesIO):
 
     def _fmt(self, field_size: int) -> str:
         try:
-            return {2: "H", 4: "L"}[field_size]
+            return {2: "H", 4: "L", 8: "Q"}[field_size]
         except KeyError:
             msg = "offset is not supported"
             raise RuntimeError(msg)