Tighten up the spec for the type attribute

[zcorpan]

Possibly the spec needs something more around the type attribute. e.g. what should happen for the empty string? application/octet-stream? foo\bar? image/gif; foo=bar?

I guess we should be consistent with what the object element does.

[tabatkins]

Yeah, I had no idea what that should be beyond "a MIME type".