=== modified file 'email_template/email_template.py'
--- email_template/email_template.py	2013-04-22 10:21:14 +0000
+++ email_template/email_template.py	2013-06-03 17:15:21 +0000
@@ -340,7 +340,8 @@
                 ctx['lang'] = self.render_template(cr, uid, template.lang, template.model, res_id, context)
             service = netsvc.LocalService(report_service)
             (result, format) = service.create(cr, uid, [res_id], {'model': template.model}, ctx)
-            result = base64.b64encode(result)
+            if 'default_composition_mode' in ctx and not ctx['default_composition_mode'] == 'mass_mail':
+                result = base64.b64encode(result)
             if not report_name:
                 report_name = report_service
             ext = "." + format