Filter possible document types on the model

When choosing a document type for an attachment, only document types
of the related record model are proposed.

document_type/models/document_type.py

@@ -15,4 +15,5 @@ class IrAttachment(models.Model):
     _inherit = 'ir.attachment'
 
     document_type_id = fields.Many2one(
-        'document.type', string='Document Type')
+        'document.type', string='Document Type',
+        domain='[("ir_model_id.model", "=", res_model)]')

document_type/views/attachment.xml

@@ -11,7 +11,9 @@
         <sheet>
           <group>
             <field name="name"/>
-            <field name="document_type_id"/>
+            <!-- Needed for the automatic domain on document_type_id -->
+            <field name="res_model" invisible="True"/>
+            <field name="document_type_id" options="{'no_create': True}"/>
             <field name="description"/>
           </group>
         </sheet>