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.

code/cms/DMSUploadField.php

@@ -57,6 +57,15 @@ class DMSUploadField extends UploadField {
 		return true;
 	}
 
+
+	public function isDisabled() {
+		return (parent::isDisabled() || !$this->isSaveable());
+	}
+
+	public function isSaveable() {
+		return (!empty($this->getRecord()->ID));
+	}
+
 	/**
 	 * Action to handle upload of a single file
 	 *