I found some great explanation here https://www.odoo.com/forum/help-1/question/where-are-document-attachments-stored-529 (Antony Lesuisse's answer) on how to set odoo to save the attachments in the file system.

I then used the following piece of code to set a default for 'parent_id' field in the attachment. (ir.attachment class)

from openerp.osv import osv, fields
from openerp.tools.translate import _
class ir_attachment(osv.osv):
    #extend document
    _inherit = "ir.attachment"
    def create(self, cr, uid, vals, context=None):
        if context is None:
            context = {}
        if not vals.get('res_model', False) and context.get('default_res_model', False):
            vals.get['res_model'] = context.get('default_res_model', False)        
        if not vals.get('parent_id', False):
            parent_id = self.pool.get('document.directory').search(cr, uid, [('ressource_type_id','=',vals.get('res_model'))])
            if parent_id and parent_id[0]:
               vals['parent_id'] = parent_id[0]
            else:
                vals['parent_id'] = self.pool.get('document.directory')._get_root_directory(cr,uid, context)
        
        return super(ir_attachment, self).create(cr, uid, vals, context)
ir_attachment()
class document_directory(osv.osv):
    _inherit = 'document.directory'
    _sql_constraints = [
        ('dir_uniq', 'unique (ressource_id,ressource_parent_type_id)',
         'The Directory name must be unique per Resource Type and Resource !'),
    ]