Strange issue with Gallery and PNG images

Just set up a new MODx Revo (2.1.1 Traditional) site last night. It’s installed on a Hostgator account; CentOS as the operating system, PHP 5.2.17/Zend 2.2.0, MySQL 5.1.52.

Installed the Gallery plugin and created a set of gallery albums. Everything went smoothly, except:

The Gallery uses a mix of JPEG and PNG images. When a user views the images in a Gallery album and clicks the Download Original Image link, the JPEG images download to the browser as expected. The PNG images, however, end up downloading as though they were text, so the user sees a raw text stream of gibberish rather than the image in the browser.

Creating a MIME-TYPE for PNG images in MODx did not help.

Any ideas?