Add check for ascii only in recipients file

tests/app/main/views/test_send.py

@@ -1529,6 +1529,36 @@ def test_check_messages_shows_over_max_row_error(
     )
 
 
+def test_special_characters_in_dvla_recipients_file_shows_error(
+    logged_in_client,
+    mock_get_users_by_service,
+    mock_get_service,
+    mock_get_service_letter_template,
+    mock_get_detailed_service_for_today,
+    fake_uuid,
+    mocker
+):
+    mocker.patch('app.main.views.send.get_page_count_for_letter', return_value=1)
+
+    mock_recipients = mocker.patch('app.utils.Spreadsheet.from_file').return_value
+    mock_recipients.as_dict = {
+            'file_name': 'invalid_characters.csv', 'data':
+            'address line 1,address line 2,address line 3,address line 4,address line 5,address line 6,postcode\r\n'\
+            'B. √Name,345 Example Street,,,,,ZM4 6HQ©'
+        }
+
+    response = logged_in_client.post(
+        url_for('main.send_messages', service_id=SERVICE_ONE_ID, template_id=fake_uuid),
+        data={'file': (None, 'invalid_characters.csv')},
+    )
+
+    assert response.status_code == 200
+    page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+    assert ' '.join(
+        page.find('div', class_='banner-dangerous').text.split()
+    ) == 'Invalid characters in the address fields within invalid_characters.csv.'
+
+
 def test_check_messages_redirects_if_no_upload_data(logged_in_client, service_one, mocker):
     checker = mocker.patch('app.main.views.send.get_check_messages_back_url', return_value='foo')
     response = logged_in_client.get(url_for(