tds/src/main/java/thredds/featurecollection/InvDatasetFeatureCollection.java
public File getFile(String remaining) {
    if (null == topDirectory) return null;
    int pos = remaining.indexOf(FILES);
    StringBuilder fname = new StringBuilder(topDirectory);
    if (!topDirectory.endsWith("/"))
      fname.append("/");
    fname.append((pos > -1) ? remaining.substring(pos + FILES.length() + 1) : remaining);
    return new File(fname.toString());
  }
public File getFile(String remaining) {
    if (null == topDirectory) return null;
    int pos = remaining.indexOf(FILES);
    StringBuilder fname = new StringBuilder(topDirectory);
    if (!topDirectory.endsWith("/"))
      fname.append("/");
    fname.append((pos > -1) ? remaining.substring(pos + FILES.length() + 1) : remaining);
    return new File(fname.toString());
  }
this says that a File URL has to be topDirectory + [FILES/ +] + match.remaining