What did you do?

9-bit precision JPEG 2000 codestream with a .txt extension: 9bit.txt (rename to 9bit.j2k)

with Image.open("9bit.j2k") as im:
    assert im.mode == "I;16"
    assert im.size == (128, 128)

What did you expect to happen?

No exceptions

What actually happened?

AssertionError: assert 'L' == 'I;16'

What are your OS, Python and Pillow versions?

• OS: Ubuntu 22.04
• Python: 3.12
• Pillow: Current main 19cd94b