NEW UploadField lists all files, shows path info

It doesn't make a lot of sense to limit the listing
to assets/Uploads/, which is the default set through FileField->folderName.
Showing all files regardless of folder makes them easier
to find, users can still opt-in to filtering by folder
through the TreeDropdownField.
Merge pull request #2734 from chillu/pulls/uploadfield-cols

NEW UploadField lists all files, shows path info