Fixed ICNS file pointer saving

Tests/test_file_icns.py

@@ -55,6 +55,19 @@ def test_save_append_images(tmp_path):
             assert_image_equal(reread, provided_im)
 
 
+@pytest.mark.skipif(sys.platform != "darwin", reason="Requires macOS")
+def test_save_fp():
+    fp = io.BytesIO()
+
+    with Image.open(TEST_FILE) as im:
+        im.save(fp, format="ICNS")
+
+    with Image.open(fp) as reread:
+        assert reread.mode == "RGBA"
+        assert reread.size == (1024, 1024)
+        assert reread.format == "ICNS"
+
+
 def test_sizes():
     # Check that we can load all of the sizes, and that the final pixel
     # dimensions are as expected

src/PIL/IcnsImagePlugin.py

@@ -337,6 +337,10 @@ def _save(im, fp, filename):
 
         # iconutil -c icns -o {} {}
 
+        fp_only = not filename
+        if fp_only:
+            f, filename = tempfile.mkstemp(".icns")
+            os.close(f)
         convert_cmd = ["iconutil", "-c", "icns", "-o", filename, iconset]
         convert_proc = subprocess.Popen(
             convert_cmd, stdout=subprocess.PIPE, stderr=subprocess.DEVNULL
@@ -349,6 +353,10 @@ def _save(im, fp, filename):
         if retcode:
             raise subprocess.CalledProcessError(retcode, convert_cmd)
 
+        if fp_only:
+            with open(filename, "rb") as f:
+                fp.write(f.read())
+
 
 Image.register_open(IcnsImageFile.format, IcnsImageFile, lambda x: x[:4] == b"icns")
 Image.register_extension(IcnsImageFile.format, ".icns")