Save all MODES as all SAVE formats

[CTimmerman]

Fixes #2663
Fixes #9428

Changes proposed in this pull request:

• Save all MODES as all SAVE formats.
• Accept extensions in format for unnamed file object saving of mode L in BW etc.
• Document name feature of file pointer if user still prefers to not use the format parameter for nameless file objects.

[CTimmerman]

I don't know why only amazon-2-amd64 failed to load the jpeg2k plugin, but that might relate to #9432.

Also, mode CMYK fails to save in Jpeg2k plugin, but only on the Ubuntu test machines, so on sys.platform "linux" i also convert CMYK to RGBA when saving JPEG2000.

[radarhere]

I don't know why only amazon-2-amd64 failed to load the jpeg2k plugin

It would be because OpenJPEG is not installed - https://github.com/python-pillow/Pillow/actions/runs/22418319033/job/64909670542?pr=9447#step:6:694

External libraries are not always installed by users in the real world either, and tests should accommodate this by avoiding checking that functionality with something like

Pillow/Tests/test_image.py

Line 191 in e96c5a5

if ext == ".jp2" and not features.check_codec("jpg_2000"):

or

Pillow/Tests/test_image.py

Line 861 in e96c5a5

@skip_unless_feature("webp")

[CTimmerman]

Thanks for the touch up. I assume since the modebase uses the same color space as the mode, it doesn't need the dither argument? checks implementation Nope, that only returns L or RGB so should get dithered as well.

[CTimmerman]

That one fail is a pipeline glitch.

[CTimmerman]

I see the struct error for PCX with a zero size is 'H' format requires 0 <= number <= 65535 which is confusing as the size was zero, so i'll restore the ValueError code.