Steps to reproduce the issue

Install Joomla 4.0.0 beta 1
login to /administrator/ as super admin
Navigate to the media manager
Upload some PNG images

Expected result

Image uploads and shows a preview

Actual result

Image uploads and shows the representation of a transparent square image

The images are actually uploaded and doing a hard refresh of the page the thumbnails correctly show again.

System information (as much as possible)

Does the same thing in Safari and Google Chrome on Mac

Additional comments

Same happens with drag and dropping image on the screen

Nothing special about the PNG's - here is one I tested with