[IMP] Option to permit custom dump for backup

This improvement is necessary for future migrations.

It permits the user to get the original PostgreSQL custom dump that we
use to have until version 8.0.

@rim-odoo 
Yes, since the original extension was ```.dump```, I guess the intention was to make something a bit like OpenOffice with the ```.odt``` extension which is actually a ZIP.

Making our own extension is good for the migration since I can detect if someone is going to upload a kind of backup they shouldn't use for migration. ```dump``` can mean anything and the MIME detection of the file is ZIP anyway.

So that's why I need something safe: a brand new extension for our special backup format.