What causes a png to display as a jpg?

Just curious, because I noticed that when I was posting the Oath pbf images, some cards like Garrison posted as pngs, with rounded edges, while most got converted to jpgs, with the corners filled in white (which looks off in dark mode).

I’m not bothered by the jpg conversion, but I am curious as to what the trigger is. The images that got converted and the ones that didn’t are the same size.

Apparently Discourse converts png to jpg if it thinks the optimisation is worthwhile. Also apparently, the value of ‘worthwhile’ is a site setting that can be adjusted. See eg Why default to converting PNG to JPG? - #19 by codinghorror - feature - Discourse Meta (and that whole thread).