Use im.has_transparency_data for webp._save_all

Also:
remove _VALID_WEBP_MODES and _VALID_WEBP_LEGACY_MODES consts
RGBX is not faster RGB since demands more bandwidth
Do not convert to str paths in tests

Tests/test_file_webp.py

@@ -72,7 +72,7 @@ class TestFileWebp:
     def _roundtrip(
         self, tmp_path: Path, mode: str, epsilon: float, args: dict[str, Any] = {}
     ) -> None:
-        temp_file = str(tmp_path / "temp.webp")
+        temp_file = tmp_path / "temp.webp"
 
         hopper(mode).save(temp_file, **args)
         with Image.open(temp_file) as image:
@@ -116,7 +116,7 @@ class TestFileWebp:
         assert buffer_no_args.getbuffer() != buffer_method.getbuffer()
 
     def test_save_all(self, tmp_path: Path) -> None:
-        temp_file = str(tmp_path / "temp.webp")
+        temp_file = tmp_path / "temp.webp"
         im = Image.new("RGB", (1, 1))
         im2 = Image.new("RGB", (1, 1), "#f00")
         im.save(temp_file, save_all=True, append_images=[im2])
@@ -151,18 +151,16 @@ class TestFileWebp:
 
     @pytest.mark.skipif(sys.maxsize <= 2**32, reason="Requires 64-bit system")
     def test_write_encoding_error_message(self, tmp_path: Path) -> None:
-        temp_file = str(tmp_path / "temp.webp")
         im = Image.new("RGB", (15000, 15000))
         with pytest.raises(ValueError) as e:
-            im.save(temp_file, method=0)
+            im.save(tmp_path / "temp.webp", method=0)
         assert str(e.value) == "encoding error 6"
 
     @pytest.mark.skipif(sys.maxsize <= 2**32, reason="Requires 64-bit system")
     def test_write_encoding_error_bad_dimension(self, tmp_path: Path) -> None:
-        temp_file = str(tmp_path / "temp.webp")
         im = Image.new("L", (16384, 16384))
         with pytest.raises(ValueError) as e:
-            im.save(temp_file)
+            im.save(tmp_path / "temp.webp")
         assert (
             str(e.value)
             == "encoding error 5: Image size exceeds WebP limit of 16383 pixels"
@@ -187,9 +185,8 @@ class TestFileWebp:
     def test_no_resource_warning(self, tmp_path: Path) -> None:
         file_path = "Tests/images/hopper.webp"
         with Image.open(file_path) as image:
-            temp_file = str(tmp_path / "temp.webp")
             with warnings.catch_warnings():
-                image.save(temp_file)
+                image.save(tmp_path / "temp.webp")
 
     def test_file_pointer_could_be_reused(self) -> None:
         file_path = "Tests/images/hopper.webp"
@@ -204,15 +201,16 @@ class TestFileWebp:
     def test_invalid_background(
         self, background: int | tuple[int, ...], tmp_path: Path
     ) -> None:
-        temp_file = str(tmp_path / "temp.webp")
+        temp_file = tmp_path / "temp.webp"
         im = hopper()
         with pytest.raises(OSError):
             im.save(temp_file, save_all=True, append_images=[im], background=background)
 
     def test_background_from_gif(self, tmp_path: Path) -> None:
+        out_webp = tmp_path / "temp.webp"
+        
         # Save L mode GIF with background
         with Image.open("Tests/images/no_palette_with_background.gif") as im:
-            out_webp = str(tmp_path / "temp.webp")
             im.save(out_webp, save_all=True)
 
         # Save P mode GIF with background
@@ -220,11 +218,10 @@ class TestFileWebp:
             original_value = im.convert("RGB").getpixel((1, 1))
 
             # Save as WEBP
-            out_webp = str(tmp_path / "temp.webp")
             im.save(out_webp, save_all=True)
 
         # Save as GIF
-        out_gif = str(tmp_path / "temp.gif")
+        out_gif = tmp_path / "temp.gif"
         with Image.open(out_webp) as im:
             im.save(out_gif)
 
@@ -234,10 +231,10 @@ class TestFileWebp:
         assert difference < 5
 
     def test_duration(self, tmp_path: Path) -> None:
+        out_webp = tmp_path / "temp.webp"
+
         with Image.open("Tests/images/dispose_bgnd.gif") as im:
             assert im.info["duration"] == 1000
-
-            out_webp = str(tmp_path / "temp.webp")
             im.save(out_webp, save_all=True)
 
         with Image.open(out_webp) as reloaded:
@@ -245,7 +242,7 @@ class TestFileWebp:
             assert reloaded.info["duration"] == 1000
 
     def test_roundtrip_rgba_palette(self, tmp_path: Path) -> None:
-        temp_file = str(tmp_path / "temp.webp")
+        temp_file = tmp_path / "temp.webp"
         im = Image.new("RGBA", (1, 1)).convert("P")
         assert im.mode == "P"
         assert im.palette is not None

src/PIL/WebPImagePlugin.py

@@ -13,10 +13,6 @@ except ImportError:
     SUPPORTED = False
 
 
-_VALID_WEBP_MODES = {"RGBX": True, "RGBA": True, "RGB": True}
-
-_VALID_WEBP_LEGACY_MODES = {"RGB": True, "RGBA": True}
-
 _VP8_MODES_BY_IDENTIFIER = {
     b"VP8 ": "RGB",
     b"VP8X": "RGBA",
@@ -247,27 +243,16 @@ def _save_all(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
 
                 # Make sure image mode is supported
                 frame = ims
-                rawmode = ims.mode
-                if ims.mode not in _VALID_WEBP_MODES:
-                    alpha = (
-                        "A" in ims.mode
-                        or "a" in ims.mode
-                        or (ims.mode == "P" and "A" in ims.im.getpalettemode())
-                    )
-                    rawmode = "RGBA" if alpha else "RGB"
-                    frame = ims.convert(rawmode)
-
-                if rawmode == "RGB":
-                    # For faster conversion, use RGBX
-                    rawmode = "RGBX"
+                if frame.mode not in ("RGBX", "RGBA", "RGB"):
+                    frame = frame.convert("RGBA" if im.has_transparency_data else "RGB")
 
                 # Append the frame to the animation encoder
                 enc.add(
-                    frame.tobytes("raw", rawmode),
+                    frame.tobytes(),
                     round(timestamp),
                     frame.size[0],
                     frame.size[1],
-                    rawmode,
+                    frame.mode,
                     lossless,
                     quality,
                     alpha_quality,
@@ -310,7 +295,7 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     method = im.encoderinfo.get("method", 4)
     exact = 1 if im.encoderinfo.get("exact") else 0
 
-    if im.mode not in _VALID_WEBP_LEGACY_MODES:
+    if im.mode not in ("RGB", "RGBA"):
         im = im.convert("RGBA" if im.has_transparency_data else "RGB")
 
     data = _webp.WebPEncode(