Fetch: Provide example for File Download

[mmocny]

I tried to point someone to this repo who was asking a question about an easier replacement for XHR+FileSystem for File Downloads, but there seems not be an example for this use case specifically.

[dgraham]

I don't have a download example readily available that we can use here, but I would merge a pull request if someone else has an example.

[gabrielboucher]

+1

[nic]

voilà!!!

    fetch(url_to_file, {
      method : 'GET'
    })
      .then(result => result.blob())
      .then(result => {
          var a = document.createElement("a");
          document.body.appendChild(a);
          a.style = "display: none";
          var blob = new Blob([result], {type: "application/pdf"});
          var url = window.URL.createObjectURL(blob);
          a.href = url;
          a.download = "filename_here.pdf";
          a.click();
          window.URL.revokeObjectURL(url);
        }
      )