Do not load image during save if file extension is unknown

[radarhere]

Pillow/src/PIL/Image.py

Lines 2513 to 2532 in b8abded

	# may mutate self!
	self._ensure_mutable()
	save_all = params.pop("save_all", False)
	self.encoderinfo = {**getattr(self, "encoderinfo", {}), **params}
	self.encoderconfig: tuple[Any, ...] = ()
	preinit()
	filename_ext = os.path.splitext(filename)[1].lower()
	ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
	if not format:
	if ext not in EXTENSION:
	init()
	try:
	format = EXTENSION[ext]
	except KeyError as e:
	msg = f"unknown file extension: {ext}"
	raise ValueError(msg) from e

where

Pillow/src/PIL/Image.py

Lines 636 to 640 in b8abded

	def _ensure_mutable(self) -> None:
	if self.readonly:
	self._copy()
	else:
	self.load()

If the ValueError is hit, there is no need to have loaded the image first. Avoiding that is a performance improvement for this situation.

A test needed to be updated to continue testing a failed load operation, rather than an unknown file extension.