Problem/Motivation
The RSS format has the <enclosure> tag for "attaching" media to an item. It is typically used for the main image of an article or the audio file of a podcast issue.
Views has an RSS Enclosure formatter but it looks like it cannot really be used. Only the fields selected in the RssFields plugin settings form are rendered.
Proposed resolution
Add an optional "Enclosure field" option to RssFields.
Remaining tasks
Add unit tests for the enclosure tag.