Converting "I" image to RGB issue

[Lucuz91]

Hi all,

I am trying to convert a png image to a jpeg.
The image is opened in "I" mode, but when I convert to RGB and save it in jpeg the image is very different from the original.

• OS: Ubuntu 20.04
• Python: 3.8.5
• Pillow: 8.3.1

from PIL import Image
img = Image.open('black_bands.png')
img2 = img.convert('RGB')
img2.save('black_bands_out.jpg')

Original image

Converted image

[radarhere]

This is something of a known issue. #4044 found this converting to RGBA, and #3159 talks about this problem more generally.

[radarhere]

Closing as part of #3159