Stop overwriting S3 metadata with empty filename

When you upload a CSV the check page takes the file name from the URL
and writes it to the S3 metadata for that file.

This also happens when you go to the .png version of this page.

The .png version of the page doesn’t have the filename in the URL. So it
re-writes the S3 metadata with an empty filename.

This means that all letter jobs sent recently have no file names. This
commit fixes this problem.

tests/app/main/views/test_send.py

@@ -666,6 +666,39 @@ def test_upload_valid_csv_shows_preview_and_table(
             assert normalize_spaces(str(row.select('td')[index + 1])) == cell
 
 
+def test_upload_valid_csv_only_sets_meta_if_filename_known(
+    client_request,
+    mocker,
+    mock_get_live_service,
+    mock_get_service_letter_template,
+    mock_get_users_by_service,
+    mock_get_service_statistics,
+    mock_get_job_doesnt_exist,
+    mock_s3_set_metadata,
+    fake_uuid,
+):
+
+    mocker.patch('app.main.views.send.s3download', return_value="""
+        addressline1, addressline2, postcode
+        House       , 1 Street    , SW1A 1AA
+    """)
+    mocker.patch(
+        'app.main.views.send.TemplatePreview.from_utils_template',
+        return_value='foo'
+    )
+
+    client_request.get(
+        'main.check_messages_preview',
+        service_id=SERVICE_ONE_ID,
+        template_id=fake_uuid,
+        upload_id=fake_uuid,
+        filetype='pdf',
+        _test_page_title=False,
+    )
+
+    assert len(mock_s3_set_metadata.call_args_list) == 0
+
+
 def test_file_name_truncated_to_fit_in_s3_metadata(
     client_request,
     mocker,