Possibility to use Postgres as a backend.

It would be nice to support Postgres as one of the backends for attachments using "large objects".
More info about them can be found here: http://www.postgresql.org/docs/9.3/static/largeobjects.html

Not sure if this should be included in this project or should go as a separate gem. 

Please review this.
Cannot even answer to that :) I really do not like this code. i guess table name should be passed inside backend initialiser. Gem could help user and provide template for Rails migration but should not handle table creation.