fix upload multiple selection display

[pxpm]

WHY

fixes: #5623

BEFORE - What was wrong? What was happening before this PR?

When deleting all previous files from the input and selecting some new files to be uploaded, the new selected files wouldn't be displayed in the UI, making developers think the files may not have been selected, when they actually were selected.

AFTER - What is happening after this PR?

We properly initialize the container, so the selected files show on the UI.

HOW

How did you achieve that, in technical terms?

Updated the selector and re-initialized the existingFiles variable on each file change.

Is it a breaking change?

No

How can we test the before & after?

1. Add a few files and save.
2. Go to edit, **remove all files**, do not save, and select a few new files to update.
  - You will notice that no new file selection is being reflected on the GUI (if you remove a few, not all, then the selection GUI will reflect fine.)
  - But when you click save, new files are saved without any issue and displayed perfectly.