According to this post: http://ifpeople.wordpress.com/2010/10/20/plone-4-best-yet-of-the-best-cms/
There is words about data storage:
Plone 4′s capacity to handle very large files has improved drastically since all file data is now stored on the file system rather than in the database. This enhances the ability of Plone to scale to handle huge content repositories out of the box!
I'm not plone user. What the meaning of that words? Is it flat file database?