Fix import/export no additional file

modules/problem.js

@@ -257,6 +257,7 @@ app.get('/problem/:id/export', async (req, res) => {
       limit_and_hint: problem.limit_and_hint,
       time_limit: problem.time_limit,
       memory_limit: problem.memory_limit,
+      additional_file_id: problem.additional_file_id,
       file_io: problem.file_io,
       file_io_input_name: problem.file_io_input_name,
       file_io_output_name: problem.file_io_output_name,
@@ -465,6 +466,11 @@ app.post('/problem/:id/import', async (req, res) => {
       let data = await download(req.body.url + (req.body.url.endsWith('/') ? 'testdata/download' : '/testdata/download'));
       await fs.writeFileAsync(tmpFile.path, data);
       await problem.updateTestdata(tmpFile.path, await res.locals.user.hasPrivilege('manage_problem'));
+      if (json.obj.additional_file_id != null) {
+        let additional_file = await download(req.body.url + (req.body.url.endsWith('/') ? 'download/additional_file' : '/download/additional_file'));
+        await fs.writeFileAsync(tmpFile.path, additional_file);
+        await problem.updateFile(tmpFile.path, 'additional_file', await res.locals.user.hasPrivilege('manage_problem'));
+      }
     } catch (e) {
       syzoj.log(e);
     }