With SS3 RC1 and RC2 can't upload images for dataobjects.

<?php
class MapsPage extends Page {
  	static $has_many = array (
		'SmallMaps' => 'SmallMap'
	);
	public function getCMSFields() {
		$fields = parent::getCMSFields();

		$gridField = new GridField("SmallMaps", "Maps list:", $this->SmallMaps(), GridFieldConfig_RelationEditor::create());
    		$fields->addFieldToTab("Root.Maps", $gridField);
		
		return $fields;
	}
}

<?php
class SmallMap extends DataObject {
	public static $has_one = array(
		'MapThumbnail' => 'Image',
		'MapsPages' => 'MapsPage'
	);

When I select a SmallMap from MapsPage GridField and try to Upload a MapThumbnail Image from computer the UploadField shows a "not found" error.
Adding a picture from already uploaded files works ok.

The POST request that returns 404 is ss3/admin/pages/edit/EditForm/field/SmallMaps/item/9/ItemEditForm/field/MapThumbnail/upload

Uploading from ModelAdmin for the same DataObject works just fine.

Is there something missing from my code or could this be a bug?

Look at this thread http://www.silverstripe.org/general-questions/show/20011?start=0
Definitely there is a problem with SS3.