mode/file-manager: Run URL-DECODE on a URL in OPEN-FILE

The problem this PR fixes: If you open a file (e.g. a PDF) residing in a directory which contains spaces or unicode characters and then invoke `open-file` again, you will see the directory percent-encoded and no hints will be available. This is after the patch is applied:  Currently you can see `/home/vasily/Downloads/Telegram%20Desktop`