At Admin Panel -> Settings -> Switches there's a "bulk uploader" switch. This determins the interface for uploading file attachments to topics, posts and private messages. If the bulk uploader switch is on, a fancy javascript-based uploader allows the submitter to select as many files as they want. Once they select their files it starts uploading and displays a progress bar. When the files have finished uploading, the submitter can add a caption or specify the sort order.

If the bulk uploader is switched off, uploads use a simple HTML file field. No progress bar is provided and captions and sort order cannot be added except by previewing the submission or coming back to edit later.

Technical note: The bulk uploader component uses the jquery ui version from http://blueimp.github.io/jQuery-File-Upload/