Fix thumbnail generation crash after resizing

[metalerk]

Fixes #1976

Fix thumbnail generation crash when Pillow drops image.format after resizing.
Preserve original image_format in upload pipeline and fall back safely to PNG when missing.

[rafalp]

I appreciate you contributing to Misago, but can you please finish addressing one PR review before opening the next one? I've left you bunch of comments in #1980 that still need addressing. Thanks!

[rafalp]

This is wrong way to go about this. Pillow already knows what format image is, via image.format attribute. It just loses this attribute when it downscales initial image if it's too big. We should find a way to keep this initial image.format attribute instead of introducing extra logic.

[rafalp]

The good way to go about this is:

Add image_format: str option to generate_attachment_thumbnail.

Update generate_attachment_thumbnail calls to pass image_format from outside.

[metalerk]

The good way to go about this is:

Add image_format: str option to generate_attachment_thumbnail.

Update generate_attachment_thumbnail calls to pass image_format from outside.

Thanks for the hint! I'll update accordingly

[rafalp]

Needs two tests:

• Uploaded attachment with width greater than max_width
• Ditto, but height greater than max_height

[coveralls]

coverage: 96.943% (+0.005%) from 96.938%
when pulling 8792d1a on metalerk:attachment-image-size-breaks-thumbnails
into 1524809 on rafalp:main.

[rafalp]

The function that you should test is store_uploaded_file, not generate_attachment_thumbnail

[metalerk]

Hi @rafalp
I've been quite busy these days but I'll try to finish it this week. My apologies for the delay.

[rafalp]

Merged as part of #2013, thank you <3