FIX Don't show the upload field when inappropriate

This fix is quite ugly, but in theory, it's fine. By ugly,
  I mean the interface is kind of weird without it.
Because 3.1 allows uploading when a DataObject doesn't exist,
  we need to override the new defaults to not allow such if
  the DO isn't saved yet.
Merge pull request #24 from ss23/master

FIX Don't show the upload field when inappropriate