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I've often uploaded png files to i.imgur to display on Stack Exchange.

There has been an issue for some time where it will mess up the smaller m versions making alphas black, but today I'm seeing a new behaviour.
Trying to upload a png as an example to a user, I send png, i.imgur shows the uploaded ref as a jpg.

It seems to only happen if there is actually no alpha channel on the png… but not always. I'm confused.
It seems to be fine from a Mac screenshot but not from Photoshop. I've been a Photoshop user for 25 years or more & though not a graphics pro, I'm pretty sure I can save a file in the correct format. I'm not savvy enough to be able to distinguish between the two.
*See below - possibly file size related, not presence of alpha channel/transparency.

Click through any of these for full size.
Alpha Mac screenshot - arrives as png (with broken m version inlined).
enter image description here

No alpha - arrives as jpg
enter image description here

Same image with 'forced' alpha channel from photoshop - but still arrives as jpg

enter image description here

With a true transparency channel added (blob in the middle), also arrives as jpg

enter image description here

This last is truly broken as a jpg.
It's now literally parsed as a white circle, as there's no alpha/transparency. Fine on a white page, but not if I'm trying to inline an image the end user is supposed to download from here to test.

Case in point is I'm trying to upload a known-quantity png to demonstrate 8-bit banding in an image. (Uncontrolled) jpg is just so completely & utterly the wrong file type for this exercise. It's not only banding, but you can almost see the jpg 'chunkiness' leap out in the converted version, totally destroying my attempted demo.

Final test as 100% jpg upload…

enter image description here

It reduces quality slightly, but actually doesn't destroy my test file, so I guess this will have to do for now. In this particular instance I'm not dependant on it having alpha, just being of approximately 'known' quality.

*From comments
This could be based simply on file size. I've tested posting various screenshots with alpha, though it's tough to guess exactly how big any picture is going to be - & somewhere around 1MB it seems the file gets converted. I don't have a hard figure, just because of the large variable in screenshot file sizes depending on image size & pictorial content.

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