Why do you need that file path? You can use export field contents to export and open a copy of the file (export to the temporary folder to keep the copies from cluttering up the client computer's hard drive). This will work for all FileMaker clients whether local or remote.
I need the file path so that the user can quickly open a windows explorer window and see the files stored in the database, by clicking a button which opens this url
I don't think that a folder set up as the remote storage location can be opened in that way. One of the reasons for setting up such a location is that the files in the folder are protected, with older versions, you set up a shared director using the "store a reference" option but users that could add image files had to have Write access to the folder and this risks the user being able to delete files from the folder that you don't want them to delete.
But the remote access location is a none filepath specified when you set up remote storage and will be the same for all files stored to that remote location.