When I attach a PNG image to my message, the semi-transparent parts become black on the thumbnail preview and on the image viewer. I can still download the original picture, but that’s quite annoying.
Steps to reproduce
I had this issue with Mattermost 5.37.0 and I upgraded today to Mattermost 5.38.0 and still have the same bug. Before 5.37.0, I didn’t had this issue. I’m using Ubuntu Server 18.04.5 LTS.
To reproduce it, you just have to upload a PNG image with semi-transparent zone (any window screenshot from the macOS native Capture.app for example)