We need the Save As box popup and let the user choose which folder to save their file to.

This isn't a choice you have. The user's browser will determine where the file is saved on the client. Some (most?) browsers allow the user to select to always prompt or to have a default where all downloads are saved.

As others have said, you can instruct the browser to treat the resource as a file to download instead of displaying the contents using the Content-disposition: HTTP Header but you cannot instruct the browser what to do with that file.