title: Generated excel file does not have extension
body: <p>I am trying to generate excel file using the following approach:</p>
  <pre><code>private static final String CONTENT_TYPE = &quot;multipart/form-data&quot;;
  private static final String FILE_NAME = &quot;DemoReport&quot;;
  private static final String FILE_EXTENSION = &quot;.xlsx&quot;;
  
  public MultipartFile export(final UUID uuid) throws IOException {
      final XSSFWorkbook workbook = new XSSFWorkbook();
      XSSFSheet sheet = workbook.createSheet(&quot;Report&quot;);
      int rowCount = 0;
  
      final List&lt;ProductDTO&gt; productList = productService
              .findAllByCategory(categoryUuid);
  
      writeTitles(sheet.createRow(rowCount++), titles); // private method
  
      // code omitted
  
      final File outputFile = File.createTempFile(FILE_NAME, FILE_EXTENSION);
  
      try (FileOutputStream outputStream = new FileOutputStream(outputFile)) {
          workbook.write(outputStream);
      } catch (IOException e) {
          LoggingUtils.error(&quot;error... &quot;, e);
      }
  
      final FileInputStream input = new FileInputStream(outputFile);
  
      final String fileName = FILE_NAME.concat(FILE_EXTENSION);
      return new MockMultipartFile(fileName,
              fileName, CONTENT_TYPE, IOUtils.toByteArray(input));
  }
  </code></pre>
  <p>However, the generated file has no extension (its type is File). So, what is the problem that causes this and how to fix?</p>