Fixed writing bytes as ASCII tag

Tests/test_file_tiff_metadata.py

@@ -185,6 +185,22 @@ def test_iptc(tmp_path):
         im.save(out)
 
 
+def test_writing_bytes_to_ascii(tmp_path):
+    im = hopper()
+    info = TiffImagePlugin.ImageFileDirectory_v2()
+
+    tag = TiffTags.TAGS_V2[271]
+    assert tag.type == TiffTags.ASCII
+
+    info[271] = b"test"
+
+    out = str(tmp_path / "temp.tiff")
+    im.save(out, tiffinfo=info)
+
+    with Image.open(out) as reloaded:
+        assert reloaded.tag_v2[271] == "test"
+
+
 def test_undefined_zero(tmp_path):
     # Check that the tag has not been changed since this test was created
     tag = TiffTags.TAGS_V2[45059]

src/PIL/TiffImagePlugin.py

@@ -727,7 +727,9 @@ class ImageFileDirectory_v2(MutableMapping):
     @_register_writer(2)
     def write_string(self, value):
         # remerge of https://github.com/python-pillow/Pillow/pull/1416
-        return b"" + value.encode("ascii", "replace") + b"\0"
+        if not isinstance(value, bytes):
+            value = value.encode("ascii", "replace")
+        return value + b"\0"
 
     @_register_loader(5, 8)
     def load_rational(self, data, legacy_api=True):