Replace sys.stdout with sys.stdout.buffer when saving

Tests/test_file_png.py

@@ -1,4 +1,5 @@
 import re
+import sys
 import zlib
 from io import BytesIO
 
@@ -10,6 +11,7 @@ from .helper import (
     PillowLeakTestCase,
     assert_image,
     assert_image_equal,
+    assert_image_equal_tofile,
     hopper,
     is_big_endian,
     is_win32,
@@ -711,6 +713,23 @@ class TestFilePng:
             with pytest.raises(EOFError):
                 im.seek(1)
 
+    def test_save_stdout(self):
+        old_stdout = sys.stdout.buffer
+
+        class MyStdOut:
+            buffer = BytesIO()
+
+        sys.stdout = mystdout = MyStdOut()
+
+        with Image.open(TEST_PNG_FILE) as im:
+            im.save(sys.stdout, "PNG")
+
+        # Reset stdout
+        sys.stdout = old_stdout
+
+        reloaded = Image.open(mystdout.buffer)
+        assert_image_equal_tofile(reloaded, TEST_PNG_FILE)
+
 
 @pytest.mark.skipif(is_win32(), reason="Requires Unix or macOS")
 @skip_unless_feature("zlib")

src/PIL/Image.py

@@ -2135,6 +2135,8 @@ class Image:
         elif isinstance(fp, Path):
             filename = str(fp)
             open_fp = True
+        elif fp == sys.stdout:
+            fp = sys.stdout.buffer
         if not filename and hasattr(fp, "name") and isPath(fp.name):
             # only set the name for metadata purposes
             filename = fp.name

src/PIL/ImageFile.py

@@ -493,7 +493,7 @@ def _save(im, fp, tile, bufsize=0):
     # But, it would need at least the image size in most cases. RawEncode is
     # a tricky case.
     bufsize = max(MAXBLOCK, bufsize, im.size[0] * 4)  # see RawEncode.c
-    if fp == sys.stdout:
+    if fp == sys.stdout.buffer:
         fp.flush()
         return
     try: