fix(mailer/models): __str__ method of Attachment returns string

jdav_web/mailer/models.py

@@ -232,7 +232,7 @@ class Attachment(CommonModel):
                             max_upload_size=10)
 
     def __str__(self):
-        return os.path.basename(self.f.name) if self.f.name else _("Empty")
+        return os.path.basename(self.f.name) if self.f.name else str(_("Empty"))
 
     class Meta:
         verbose_name = _('attachment')

jdav_web/mailer/tests/models.py

@@ -267,6 +267,5 @@ class AttachmentTestCase(BasicMailerTestCase):
         self.attachment.f.name = 'attachments/test_document.pdf'
         self.assertEqual(str(self.attachment), 'test_document.pdf')
 
-    @skip('Fails with TypeError: __str__ returns a lazy translation object, but must return a string.')
     def test_str_without_file(self):
         self.assertEqual(str(self.attachment), _('Empty'))