[FIX] mail: only register as main attachment if main not already set

Commit a26496b6e7 registered an attachment added through the widget as record's
message_main_attachment.
This to make the OCR feature available for such attachments.
However we copy more closely the behaviour in message_post,
which only puts the attachment as main if there isn't already a main.

opw 1950403

addons/mail/models/ir_attachment.py

@@ -13,9 +13,9 @@ class IrAttachment(models.Model):
         """
         super(IrAttachment, self)._post_add_create()
         for record in self:
-            record.register_as_main_attachment()
+            record.register_as_main_attachment(force=False)
 
-    def register_as_main_attachment(self):
+    def register_as_main_attachment(self, force=True):
         """ Registers this attachment as the main one of the model it is
         attached to.
         """
@@ -24,4 +24,5 @@ class IrAttachment(models.Model):
         # message_main_attachment_id field can be empty, that's why we compare to False;
         # we are just checking that it exists on the model before writing it
         if related_record and hasattr(related_record, 'message_main_attachment_id'):
-            related_record.message_main_attachment_id = self
+            if force or not related_record.message_main_attachment_id:
+                related_record.message_main_attachment_id = self