Do not prematurely return in ImageFile when saving to stdout

Tests/test_file_ppm.py

@@ -1,3 +1,6 @@
+import sys
+from io import BytesIO
+
 import pytest
 
 from PIL import Image
@@ -80,3 +83,30 @@ def test_mimetypes(tmp_path):
         f.write("PyCMYK\n128 128\n255")
     with Image.open(path) as im:
         assert im.get_format_mimetype() == "image/x-portable-anymap"
+
+
+@pytest.mark.parametrize("buffer", (True, False))
+def test_save_stdout(buffer):
+    old_stdout = sys.stdout
+
+    if buffer:
+
+        class MyStdOut:
+            buffer = BytesIO()
+
+        mystdout = MyStdOut()
+    else:
+        mystdout = BytesIO()
+
+    sys.stdout = mystdout
+
+    with Image.open(TEST_FILE) as im:
+        im.save(sys.stdout, "PPM")
+
+    # Reset stdout
+    sys.stdout = old_stdout
+
+    if buffer:
+        mystdout = mystdout.buffer
+    with Image.open(mystdout) as reloaded:
+        assert_image_equal_tofile(reloaded, TEST_FILE)

src/PIL/ImageFile.py

@@ -493,13 +493,6 @@ 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
-    try:
-        stdout = fp == sys.stdout or fp == sys.stdout.buffer
-    except (OSError, AttributeError):
-        stdout = False
-    if stdout:
-        fp.flush()
-        return
     try:
         fh = fp.fileno()
         fp.flush()