Fix letter branding preview

Template preview requires `template_type` to be passed in now.

app/main/views/templates.py

@@ -248,7 +248,8 @@ def letter_branding_preview_image(filename):
             'The point of using Lorem Ipsum is that it has a more-or-less '
             'normal distribution of letters, as opposed to using ‘Content '
             'here, content here’, making it look like readable English.'
-        )
+        ),
+        'template_type': 'letter',
     }
     filename = None if filename == 'no-branding' else filename
 

tests/app/main/views/test_templates.py

@@ -845,7 +845,14 @@ def test_letter_branding_preview_image(
         url_for('no_cookie.letter_branding_preview_image', filename=original_filename)
     )
 
-    mocked_preview.assert_called_with(ANY, new_filename)
+    mocked_preview.assert_called_with(
+        {
+            'subject': 'An example letter',
+            'content': ANY,
+            'template_type': 'letter',
+        },
+        new_filename,
+    )
     assert resp.get_data(as_text=True) == 'foo'