It’s definitely not a Permissions issue, otherwise you’d see “Destination folder is not writable or does not exists” instead of “File validation failed”.
Apparently, there was a bug that gave the same issue back in 2.2.5 and 2.2.6. But that was related to video files, not PNGs.
I’m also using the design_config_form.xml workaround but I still can’t reproduce your issue.
Bit of a longshot but do you have Do you have a PHP module installed/enabled called phpfileinfo?
Did you build this server and install Magento 2.3 yourself?