[FIX] mass_mailing: URI too long

Since commit 6494f51171, all images in mass mailing are created as
attachments. This can lead to URI too long to be handled by some proxy,
e.g. Nginx.

opw-1853188

addons/mass_mailing/static/src/js/mass_mailing.js

@@ -38,7 +38,7 @@ FieldTextHtml.include({
         var datarecord = this._super();
         if (this.model === 'mail.mass_mailing') {
             // these fields can potentially get very long, let's remove them
-            datarecord = _.omit(datarecord, ['mailing_domain', 'contact_list_ids', 'body_html']);
+            datarecord = _.omit(datarecord, ['mailing_domain', 'contact_list_ids', 'body_html', 'attachment_ids']);
         }
         return datarecord;
     },