Using Digital File Pro, you will not be able to upload the file prior to inserting it. What you can do is to create 2 different forms, one for when the file is already uploaded and one for uploading the file.
As for having a preview, you can put a link to the preview download on the product details page, but then have a protected download page for the user to download the full version when they've purchased it. You can store what mp3s that the user has purchased and then display only those ones in the download page.