code: public static File getMountSource(Closeable handle) {
          if (handle instanceof MountHandle) { return MountHandle.class.cast(handle).getMountSource(); }
          return null;
      }
docs: Return the mount source File for a given mount handle.
  
  @param handle The handle to get the source for
  @return The mount source file or null if the handle does not have a source, or is not a MountHandle