attachment_fu: I just want an attachment

creating 2 separate models is the best bet anyway. chances are at some
point down the road not only will they need different size
constraints, they will also need differing methods and other code.
start off now with them separate and thank yourself in 3 months.

The way I prefer to do this is use attr_accessor to create an
attribute on the user model (say, :uploaded_data), that is then
checked in an after filter to create or update the associated photo
model.