Attachment URLs confusing, especially for Video

[nicpottier]

Have seen a lot of tickets over the last few weeks of people having trouble with the idea of video attachments not being able to be YouTube links.

Wonder if we should be doing some validation on the editor side when they add a URL attachment. Maybe even something as dumb as saying it has to end in mp4 or whatever other extensions we think are correct. I think this is almost always being mis-configured by people.