If you're using the Chooser and want to directly download the chosen file data, you should specify the "direct" "linkType". (Based on your sample here, you're currently using "preview" instead of "direct".)
When you use the direct link type, the Chooser result will still look similar to your sample here, but the "link" field will contain a direct link to the file data. You can then pass that link to your server and have your server download the data by using an HTTPS GET request via the HTTPS client of your choice. As long as the request is successful (i.e., check the status code for a 200), the response body will be the file data, which you can use as you wish, e.g., save it to your server's filesystem, etc. Note that these direct links expire after four hours though.