_guess_ext_from_data: data[:5] == b"#!SILK" -> data[:6] (6-byte string)
_looks_like_silk: data[:4] == b"#!SILK" -> data[:6]
The previous slices were too short to ever match the 6-byte "#!SILK"
literal, relying entirely on the "#!SILK_V3" (9-byte) and 0x02! (2-byte)
fallback paths for SILK format detection.