code: private final String getFilenameFromLabel(Context context, String pid, String dsid, Date asOfDateTime, String MIMETYPE) throws Exception {
  
          // can't get datastream label directly from datastream as this is an API-M call
          // instead get list of datastream defs, as these contain labels
          DOReader reader = m_doManager.getReader(false, context, pid);
          Datastream ds = reader.GetDatastream(dsid, asOfDateTime);
  
          return (ds == null) ? "" : ds.DSLabel;
      }
docs: Get filename based on datastream label
  
  @param context
  @param pid
  @param dsid
  @param asOfDateTime
  @param MIMETYPE
  @return
  @throws Exception