After upgrading from 2.0.2 to 2.3.2, I can't see images in the "Browse" or "Browse All" tabs while editing a post. Instead, I see the filename of the image. Posts appear normally, with images displaying properly.
No plugins are activated.
My uploads are organized into month-and year- based folders, but this problem happens whether or not "Organize my uploads into month- and year-based folders" is checked in "Options | Miscellaneous".
It happens even if I copy the images from the month-year folders into the root of the wp-content/uploads folder. (I assume that's where they go if they're not organized into month-year folders.)
It happens even if I revert the three wp-admin files that I've customized (with cosmetic changes) to the originals.
It happens in Firefox 18.104.22.168 and Safari 2.0.4 and 3.0.4 on a Mac, and Internet Explorer in Windows.