Actually I made it. But I have a problem . When I press Send An E-Mail Button first of all it was sending an email without attachment. After that the window opened and If I wish i can change the e mail template or something whatever you want.

Here is my .py code:

class ClassName(models.Model):

 _inherit = "project.task"
 def def_name(self):     

 self.ensure_one() 

 ir_model_data = self.env['ir.model.data']  

 try:     

 template_id = ir_model_data.get_object_reference('my_module_name', 'my_template_id')[1]

 except ValueError:     

 template_id = False      

 try:       

 compose_form_id = ir_model_data.get_object_reference('mail', 'email_compose_message_wizard_form')[1] 

 except ValueError:   

 compose_form_id = False  

 template = template_id and self.env['mail.template'].browse(template_id)    

 ctx = {      

 'default_model': 'project.task',  

 'default_res_id': self.ids[0],   

 'default_use_template': bool(template_id), 

 'default_template_id': template_id,       

 'default_composition_mode': 'comment',  

 'mark_so_as_sent': True,       

 'custom_layout': "mail.mail_notification_paynow",    

 'force_mail': True        }      

return {      

 'type': 'ir.actions.act_window',  

 'view_type': 'form',          

 'view_mode': 'form',       

 'res_model': 'mail.compose.message',   

 'views': [(compose_form_id, 'form')],   

 'view_id': compose_form_id,        

 'target': 'new',          

 'context': ctx,    

 }

Here is my view.xml

   <field name="model">project.task</field>   

 <field name="inherit_id" ref="project.view_task_form2"/>  

 <field name="arch" type="xml">          

 <!--Create Email-Send Button-->        

 <xpath expr ="//field[@name='stage_id']" position="before"> 

 <button name="def_name" type="object" string="Send An E-Mail"  class="oe_highlight"/>            </xpath>        </field>

And also I have template.xml there is a ref="attachment_id" in it. I think there is no problem in template.xml. My problem is I press the button it sends emails two times. One is with attachment one is not. Can someone help me ? By the way my code is similar to Sale order send mail. Thanks guys.