Today, I searched the Internet for a method of exposing the RSS <enclosure> tag for images already attached as assets of an entry in MT4 (Movable Type). It was not easy. I searched for about an hour before I came up with the following. Here's hoping Google will make it easier for the next guy. GoogleBot index!
<enclosure url="<mt:AssetURL>" length="<mt:AssetProperty property="file_size" format="0">" type="<mt:AssetMimeType>" />