to be able to delete files using the delete file behavior, there needs to be a way to get the file name. The most common way to do this is to store the files name in a database when it is uploaded, you can then create a manage files page that uses a recordset to return a listing of all the uploaded files, and offers a delete button for each one.
next to the delete button would be a hidden form element that is used to pass the file name to the delete file behavior.
If you're not storing the file name in a database, this becomes a lot more tricky to accomplish.
It would require hand coding a routine to return the files names from the upload directory. There are no WebAssist extensions available to do this, it would require php coding experience to accomplish.