新建用例-模板损坏或者乱码

1. 在新建用例，下载模板，使用testcase-template.xlsx .xlsx 后缀，文件打开是损坏的
2. 我将testcase-template.xlsx 改成 testcase-template.xls ，模板下载后，打开下载的文件，是乱码

Reproduce the download from the new-test-case flow using testcase-template.xlsx, then inspect the downloaded file and its format. Verify the template opens correctly with the .xlsx extension and that the alternative .xls download is no longer garbled.

Written by the indexing model from the issue text.