cdm/src/main/java/thredds/filesystem/MFileOS.java
static public MFileOS getExistingFile(String filename) {
    if (filename == null) return null;
    File file = new File(filename);
    if (file.exists()) return new MFileOS(file);
    return null;
  }
static public MFileOS getExistingFile(String filename) {
    if (filename == null) return null;
    File file = new File(filename);
    if (file.exists()) return new MFileOS(file);
    return null;
  }
Make MFileOS if file exists, otherwise return null
@param filename
@return MFileOS or null