Accept file id in URL, not as form parameter

In the future we need to get the metadata from the file in order to work
out what form validation rules should apply (postage is only required
for UK letters).

To start doing this we need all instances of the app accepting `post`
requests with the `file_id` in the URL, as well as in the form data (for
backwards compatibility).

app/main/views/uploads.py

@@ -339,13 +339,14 @@ def view_letter_upload_as_preview(service_id, file_id):
 
 
 @main.route("/services/<uuid:service_id>/upload-letter/send", methods=['POST'])
+@main.route("/services/<uuid:service_id>/upload-letter/send/<uuid:file_id>", methods=['POST'])
 @user_has_permissions('send_messages', restrict_admin_usage=True)
-def send_uploaded_letter(service_id):
+def send_uploaded_letter(service_id, file_id=None):
     if not (current_service.has_permission('letter') and current_service.has_permission('upload_letters')):
         abort(403)
 
-    form = LetterUploadPostageForm()
-    file_id = form.file_id.data
+    form = LetterUploadPostageForm(file_id=file_id)
+    file_id = file_id or form.file_id.data
 
     if not form.validate_on_submit():
         return uploaded_letter_preview(service_id, file_id)

tests/app/main/views/test_uploads.py

@@ -790,6 +790,65 @@ def test_send_uploaded_letter_sends_letter_and_redirects_to_notification_page(mo
     mock_send.assert_called_once_with(SERVICE_ONE_ID, 'my_file.pdf', file_id, 'first', 'address')
 
 
+@pytest.mark.parametrize('form_data', (
+    {'filename': 'my_file.pdf', 'postage': 'first'},
+    {'filename': 'my_file.pdf', 'postage': 'first', 'file_id': 'Ignored in favour of URL'},
+))
+def test_send_uploaded_letter_accepts_file_id_in_url(
+    mocker,
+    service_one,
+    client_request,
+    fake_uuid,
+    form_data,
+):
+    metadata = LetterMetadata({'filename': 'my_file.pdf', 'page_count': '1', 'status': 'valid', 'recipient': 'address'})
+
+    mocker.patch('app.main.views.uploads.get_letter_pdf_and_metadata', return_value=('file', metadata))
+    mock_send = mocker.patch('app.main.views.uploads.notification_api_client.send_precompiled_letter')
+    mocker.patch('app.main.views.uploads.get_letter_metadata', return_value=metadata)
+
+    service_one['permissions'] = ['letter', 'upload_letters']
+
+    client_request.post(
+        'main.send_uploaded_letter',
+        service_id=SERVICE_ONE_ID,
+        file_id=fake_uuid,
+        _data=form_data,
+        _expected_redirect=url_for(
+            'main.view_notification',
+            service_id=SERVICE_ONE_ID,
+            notification_id=fake_uuid,
+            _external=True
+        )
+    )
+    mock_send.assert_called_once_with(SERVICE_ONE_ID, 'my_file.pdf', fake_uuid, 'first', 'address')
+
+
+def test_send_uploaded_letter_needs_file_id_in_form_if_not_in_url(
+    mocker,
+    service_one,
+    client_request,
+    mock_template_preview,
+    fake_uuid,
+):
+    metadata = LetterMetadata({'filename': 'my_file.pdf', 'page_count': '1', 'status': 'valid', 'recipient': 'address'})
+
+    mock_send = mocker.patch('app.main.views.uploads.notification_api_client.send_precompiled_letter')
+    mocker.patch('app.main.views.uploads.get_letter_metadata', return_value=metadata)
+    mocker.patch('app.main.views.uploads.service_api_client.get_precompiled_template')
+
+    service_one['permissions'] = ['letter', 'upload_letters']
+
+    client_request.post(
+        'main.send_uploaded_letter',
+        service_id=SERVICE_ONE_ID,
+        _data={'filename': 'my_file.pdf', 'postage': 'first'},
+        _expected_status=200,
+        _expected_redirect=None,
+    )
+    assert mock_send.called is False
+
+
 @pytest.mark.parametrize('permissions', [
     ['email'],
     ['letter'],