One Drupal user had uploaded an image file named "Loons?-1.jpg". By the time the html is sent to the web browser, the filename appears as "Loons%3F-1.jpg. I assume this is some sort of UTF-8 encoding of the question mark "?". This had never been a problem prior to 7.20. Now the image cannot be displayed.
I am fortunate that there is only one filename that a member has uploaded with a "?" in it. My solution was to copy the image to the host, remove the image from Drupal, rename it on the host, and upload the new image renamed without any special characters.
None of the patches pointed to in the release notes helped.