The uploader widgets don't store files as binary data in attributes.
The files are stored on the file system. To access a file and get the data you can use in JAVA the Core API -
Core.getFileDocumentContent(IMendixObject obj) - to retrieve a java.io.InputStream with the IMendixObject that represents the FileDocument.
I hope this helps you, if not please explain why you asked this question and where you need the binary data.
Five years later... Locally, the documents are saved under a subfolder of deployment/data/files/. To know the original file name, you should have a look on the field [system$filedocument].[uuid]. If you are patient and lucky, you should fine your original document with this name.