The original file name is stored in the 'Name' attribute of the FileDocument.
We recommend you to use the method Core.getFileDocumentContent() and to use the inputstream to store a new FileDocument with the same content via Core.storeFileDocumentContent. When a user uploads a file, it will be stored on the server running the Mendix Runtime. The file name in the file system is defined by the Runtime and this file name is not for public use. When you create a file (by uploading, via Core.storeFileDocumentContent or via an own method in Java), the file is created on the server, never on the client! A file created via Core.storeFileDocumentContent can be downloaded by users.
The original file name is stored in the Name attribute as I said, but the original path is not stored in any field by the Mendix Runtime due to security reasons.