官方文档有说明：
If your API endpoint returns an image, Postman will detect and render it automatically. For binary response types, you should select “Send and download” which will let you save the response to your hard disk. You can then view it using the appropriate viewer. This gives you the flexibility to test audio files, PDFs, zip files, or anything that the API throws at you.
在Send按钮那里可以选择Send and download
