BUG CMSFileAddController: get page id for action fileexists

UploadField->fileexists() fails to check if a file exists in a certain folder (and fallsback to "assets/"), if CMSFileAddController doesn't provide a valid folder (-- and fallsback to "assets/newFolder/").

admin/assets/add/?ID=123:
$this->currentPageID() = 123
$folder->getFilename() = 'assets/Uploads/foo/bar/'

admin/assets/add/EditForm/field/AssetUploadField/fileexists?filename=test.jpg:
$this->currentPageID() = 0
$folder->getFilename() = 'assets/NewFolder/'

silverstripe/silverstripe-framework#2904