Add setting to support prepending files to progress_bar_target

This is a small change to add a setting called "prepend_files". When true, new files are prepended to the progress_bar_target instead of appended. Unless set, the current behavior to append files remains.

This prevents a common scenario that confuses users. When several files are uploading and new files are added, users often don't see the new files because they appear too far down the page. This setting ensures users get clear visual feedback.