Allow encoderinfo to be set for appended images

Tests/test_file_mpo.py

@@ -293,3 +293,15 @@ def test_save_all() -> None:
     # Test that a single frame image will not be saved as an MPO
     jpg = roundtrip(im, save_all=True)
     assert "mp" not in jpg.info
+
+
+def test_save_xmp() -> None:
+    im = Image.new("RGB", (1, 1))
+    im2 = Image.new("RGB", (1, 1), "#f00")
+    im2.encoderinfo = {"xmp": b"Second frame"}
+    im_reloaded = roundtrip(im, xmp=b"First frame", save_all=True, append_images=[im2])
+
+    assert im_reloaded.info["xmp"] == b"First frame"
+
+    im_reloaded.seek(1)
+    assert im_reloaded.info["xmp"] == b"Second frame"

src/PIL/Image.py

@@ -2565,7 +2565,7 @@ class Image:
         self._ensure_mutable()
 
         save_all = params.pop("save_all", False)
-        self.encoderinfo = params
+        self.encoderinfo = {**getattr(self, "encoderinfo", {}), **params}
         self.encoderconfig: tuple[Any, ...] = ()
 
         preinit()
@@ -2612,6 +2612,11 @@ class Image:
                 except PermissionError:
                     pass
             raise
+        finally:
+            try:
+                del self.encoderinfo
+            except AttributeError:
+                pass
         if open_fp:
             fp.close()