I’m experiencing the same problem on a CentOS 4 server running PHP 5.2.
My first thought was that WordPress couldn’t resolve the MIME type, but clicking on the image shows “File type: image/png” in the details pane.
File size and server space isn’t the issue; the files I’m uploading are tiny 512px square PNGs, and there are several GBs free on the partition.
The problem seems to have arisen since the upgrade to 4.0.
