No generic way to find out default file extensions

Created by: anjos

The current databases covered by this package base classes contain files which have original extensions to which we don't have access to unless we read the contents of the database by hand.

It would be convenient, since this is part of the database information anyways, to have it accessible in a centralized way, so I can probe for it:

>>> print db.image_extension()
'.ppm'

This would allow a person to code algorithms which are database agnostic.