Show letter preview once file is uploaded

This shows the sanitised letter preview if the file had no validation
errors or the preview with the overlay if it failed validation.

tests/app/test_template_previews.py

@@ -1,3 +1,4 @@
+import base64
 from functools import partial
 from unittest.mock import Mock
 
@@ -79,6 +80,44 @@ def test_from_database_object_makes_request(
     request_mock.assert_called_once_with(expected_url, json=data, headers=headers)
 
 
+@pytest.mark.parametrize('page_number, expected_url', [
+    ('1', 'http://localhost:9999/precompiled-preview.png?hide_notify=true'),
+    ('2', 'http://localhost:9999/precompiled-preview.png'),
+])
+def test_from_valid_pdf_file_makes_request(mocker, page_number, expected_url):
+    mocker.patch('app.template_previews.extract_page_from_pdf', return_value=b'pdf page')
+    request_mock = mocker.patch(
+        'app.template_previews.requests.post',
+        return_value=Mock(content='a', status_code='b', headers={'c': 'd'})
+    )
+
+    response = TemplatePreview.from_valid_pdf_file(b'pdf file', page_number)
+
+    assert response == ('a', 'b', {'c': 'd'}.items())
+    request_mock.assert_called_once_with(
+        expected_url,
+        data=base64.b64encode(b'pdf page').decode('utf-8'),
+        headers={'Authorization': 'Token my-secret-key'},
+    )
+
+
+def test_from_invalid_pdf_file_makes_request(mocker):
+    mocker.patch('app.template_previews.extract_page_from_pdf', return_value=b'pdf page')
+    request_mock = mocker.patch(
+        'app.template_previews.requests.post',
+        return_value=Mock(content='a', status_code='b', headers={'c': 'd'})
+    )
+
+    response = TemplatePreview.from_invalid_pdf_file(b'pdf file', '1')
+
+    assert response == ('a', 'b', {'c': 'd'}.items())
+    request_mock.assert_called_once_with(
+        'http://localhost:9999/precompiled/overlay.png?page_number=1',
+        data=b'pdf page',
+        headers={'Authorization': 'Token my-secret-key'},
+    )
+
+
 @pytest.mark.parametrize('template_type', [
     'email', 'sms'
 ])