Fix saving of non-RGB thumbnails as PNG

[mweinelt]

The PNG format does not support non-RGB color spaces, which makes pillow raise an exception when trying to save e.g. a CMYK image in PNG format.

Converting to RGB is recommended on the upstream issue, and as CMYK is a print format transparency does not need to be considered.

Closes: #10741

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[anoadragon453]

Thanks for the fix @mweinelt. I've attempted to write a test for this in #17786, but aren't actually seeing the test fail on develop, meaning I can't reproduce the issue... I've also tried just manually uploading the image from #10741 to matrix.org and thumbnail requests work fine.

Are you seeing this issue on your personal deployment? If so, what Pillow version are you using, and do you have a test image I could try?

[clokep]

I was writing some for to fix something similar and I suspect it only fails under dynamic thumbnails or something like that? Ie by default jpeg gets thumbailed as jpeg? There might be other configuration aspects too.

[anoadragon453]

Despite not being able to reproduce this, the change doesn't appear to have any downsides as indeed CMYK does not have an alpha channel. I'd say we just merge it and see if this breaks anyone's workflow.

[mweinelt]

I do not have a test image. Multiple times per day I spotted the same traceback as the author of the issue. I am not sure what images have been causing this.

Yes, private deployment on NixOS 24.05 with pillow 10.3.0.

[anoadragon453]

Thanks for confirming the details! Please open a new issue if you see the problem reoccur after this change.

It should go out in Synapse v1.118.0.