1-Delete all Document Types samples that comes with Liferay default install (Marketing, etc).
2-Create new Document Type.
3-Upload multiple files (flash uploader). In right part you can't see the combo to choose your recently created document type.
4-Add another Document Type.
5-Repeat step 3. Now you can choose your 2 document types.
With this you can't have only one document type and use with multiple uploader.