Allow invalid encoding in EXIF UserComment field

[fkleon]

I've come across multiple applications that fail to encode the EXIF UserComment field correctly (i.e. omitting the 8 byte ID code completely). In that case, ExifTool decodes the data correctly (with a warning), but imagemeta does not return any data for the field.

This seems to especially affect Python applications making use of Pillow which relies on applications implementing their own encoding for this field. One could argue that the applications are at fault here, but given "When in doubt, Exiftool is right".. here we go.

The included sample file was generated with the following script which is exactly how affected applications are setting the UserComment field:

# /// script
# requires-python = ">=3.12"
# dependencies = [
#     "pillow~=11.2.1",
# ]
# ///
from PIL import Image

def main() -> None:
    im = Image.new("1", (1, 1))
    exif = im.getexif()
    exif[0x9286] = "UserComment"
    im.save("invalid-encoding-usercomment.jpg", exif=exif)


if __name__ == "__main__":
    main()

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.91%. Comparing base (ee0de9b) to head (3ea2b56).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage   77.90%   77.91%           
=======================================
  Files          14       14           
  Lines        1711     1716    +5     
=======================================
+ Hits         1333     1337    +4     
- Misses        290      291    +1     
  Partials       88       88           

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