In your .py file you have to define the function that is called by the button "attachment_tree_view"

EDITED:

class project(osv.osv):
 _inherit = "project.project"

 def test(self, cr, uid, vals, context=None):        
     return super(project.project, self)._get_attached_docs(cr, uid, vals, context=context)

def attachment_tree_view(self,cr,uid,ids,context):
    tasks_ids = self.pool.get(project.task).search(cr,uid,[('project_id','in',ids)])
    domain = ['|','&',('res_model','=','project.project'),('res_id','in', ids),
              '&',('res_model','=','project.project'),('res_id','in', task_ids)]
    res_id = ids and ids[0] or False
    return {
        'name':_('Attachments'),
        'domain': domain,
        'res_model', 'ir.attachment',
        'type': 'ir.actions-act_window',
        'view_id': False,
        'view_mode': 'kanban,tree,form',
        'view_type': 'form',
        'limit': 80,
        'context': "{'default_res_model':'%s','default_res_id':%d}" % (self._name, res_id)
    }

 _columns = {
 'doc_count': fields.function(test, string="Number of documents attached",type='integer')
 }

...

With this you inherit original function, now just modify as you need (remember to set super if you modify before or after the original functions returns the values).

Kind regards.

I try to make this 2 buttons on the hr.timesheet.sheet view. 

My hr_timesheet.py looks like this:

# -*- coding: utf-8 -*-
import openerp
from openerp.osv import fields, osv
class hr_timesheet_sheet(osv.osv):
    _inherit = ["hr_timesheet_sheet.sheet"]

    _columns = {
        'testfield1': fields.char(''),
        'project_field': fields.many2one('project.project', 'name'),
    }
class project(osv.osv):
    _inherit = "project.project"

    def test(self, cr, uid, vals, context=None):
        return super(project.project, self)._get_attached_docs(cr, uid, vals, context=context)

    def attachment_tree_view(self, cr, uid, ids, context):
        task_ids = self.pool.get('project.task').search(cr, uid, [('project_id', 'in', ids)])
        domain = [
             '|',
             '&', ('res_model', '=', 'project.project'), ('res_id', 'in', ids),
             '&', ('res_model', '=', 'project.task'), ('res_id', 'in', task_ids)]
        res_id = ids and ids[0] or False
        return {
            'name': _('Attachments'),
            'domain': domain,
            'res_model': 'ir.attachment',
            'type': 'ir.actions.act_window',
            'view_id': False,
            'view_mode': 'kanban,tree,form',
            'view_type': 'form',
            'limit': 80,
            'context': "{'default_res_model': '%s','default_res_id': %d}" % (self._name, res_id)
        }
    _columns = {
        'doc_count': fields.function(test, string="Number of documents attached",type='integer')
    }		

My XML file like this: hr_timesheet_view_test.xml:

<?xml version="1.0" encoding="utf-8"?>
<openerp>    
    <data>        
        <record id="hr_timesheet_sheet_tree_test_buttons" model="ir.ui.view">
            <field name="name">hr.test.button.tree</field>
            <field name="model">hr_timesheet_sheet.sheet</field>
            <field name="inherit_id" ref="hr_timesheet_sheet.hr_timesheet_sheet_form"/> <!-- "hr_presence.hr_timesheet_sheet_form_presence" -->
            <field name="arch" type="xml">
                    <xpath expr="//notebook" position="before">
                       <button  class="oe_inline oe_stat_button" name="attachment_tree_view"  type="object" icon="fa-files-o">
                           <field string="Documents" name="doc_count" widget="statinfo"/>
                       </button>
                    </xpath>
            </field>
        </record>
    </data>
</openerp>

This is not working , I get the error that the field : doc_count does not exist.