[add] new helper func in document attachment model

Note that now we can use this method to retrieve metadata from attachments.

models/document_attachment.py

@@ -80,3 +80,20 @@
         attachments.unlink()
 
         return ret
+
+    @api.multi
+    def get_metadata(self):
+        """Helper function returning the document attachment metadata
+        Can be overridden to add some specifics metadata
+        """
+
+        self.ensure_one()
+
+        return {
+            "size": self.file_size,
+            "type": self.mimetype,
+            "filename": self.datas_fname,
+            "hash": ["sha1:%s" % self.checksum],
+            "encoding": "",
+            "customtype": self.type_id.name,
+        }