Comment 4 for bug 15224

I remember a discussion some time ago about this issue but don't remember what
we decided. Please correct me if I am wrong. TYPE attribute should take
precedence, if it is missing we should probably look at the HTTP headers and so
on. So in order of precedence:
  1. TYPE attribute
  2. mime type reported by server
  3. OS level file ext to mime type mapping
  4. choose plugin by file extension only

The first two are most the controversial I would guess.