WordPress Download Manager gives you a verity way of attaching files, you can upload from your pc or use server file browser to explore files in the server and attach from there or attach an URL from any remote location, also you can use WordPress Download Manager Cloud Storage add-ons if you want to keep your files in cloud storage like Amazon S3, Google Drive, Dropbox, Box.com or One Drive and serve the downloads from your site. And in WordPress download manager, you can use those cloud storage altogether and which gives you lots of free storage and bandwidth.
Attach File section is divided into three tabs named Upload, Browse and Remote.
You can upload files from your local computer by clicking “Select Files” button or by dragging files into the box. You can select one or more files at a time and click "Open". The upload will start immediately. You’ll see a progress bar for each file upload.
By this tab, you can browse files from your server and add them to your package. You need to select file browser root directory from download manager settings page. if you don’t set root directory then default root will be your WordPress base dir. Select your specific file by browsing your file structures and click the file name you want to add. An alert message will appear with "Ok" and "Cancel" buttons. Click "Ok" button to add this file and "Cancel" button otherwise. Note that, if you want multiple files in your package by this method, you have to add them by clicking one by one.
You can use remote URL as file source using this option and you can add multiple URL with the same package.
After adding files, your files will be displayed under "Attached Files" section for further modifications.
Also, you can attach a directory path to your download manager package:
You can user server directory browser to browse and select directory:
And, you can add, edit or delete any file in that directly directly from ftp or any other way, and any modification in that directory will be apply on package download automatically.
And at front-end you can show the file list of attached directory like this: