This PR makes small improvement to FilePathRule so that the entered path not only must be an internal path but also needs to be a valid/existing folder.
Testing Instructions
Access to Content -> Media, click on Options button in the toolbar
Change Path to Files Folder from images to a random string like aaa (a none existing folder)
Before patch: The change is saved
After patch: The change is not saved, there is an error message displayed Invalid field: Path to Files Folder