I've been trying out Radio3, with success in posting Tweets. Frank McPherson's "Notes on Using Radio3" is very explicit and helpful in detailing what fields do what under various scenarios. But I haven't been able to find anything about the Enclosure Field anywhere, including in this forum or in the Radio 3 docs. Wikipedia tells me that RSS enclosures are a way of attaching multimedia content to RSS feeds, so I suppose that is why the field is there. I put a URL in the field on two of my tests, but it did not appear either on Twitter or in my R3 Linkblog feed. I haven't tried it with Facebook and I don't have a WordPress installation to try it there.
I have a vague recollection that Dave wrote about enclosures recently, but I haven't been able to find it again. Can anyone shed any light on how to use this field?