[56steve]

Opening a file that starts with the XV thumbnail magic but has a missing or non-numeric size line raised a bare ValueError from the unpacking or int() call, instead of the SyntaxError that lets Image.open report UnidentifiedImageError. For example, b"P7 332\n#c\n64\n" fails this way. I now catch that and raise SyntaxError, and added a test for it.

[radarhere]

Your data starts with the identifying P7 332.

Pillow/src/PIL/XVThumbImagePlugin.py

Line 24 in 2946b9c

_MAGIC = b"P7 332"

Pillow/src/PIL/XVThumbImagePlugin.py

Lines 36 to 37 in 2946b9c

	def _accept(prefix: bytes) -> bool:
	return prefix.startswith(_MAGIC)

So it is in the XVThumb format. Why do you feel that it should raise an UnidentifiedImageError?