code: def is_dicom_file(filename):
      
      file_stream = open(filename, 'rb')
      file_stream.seek(128)
      data = file_stream.read(4)
      file_stream.close()
      if data == b'DICM':
          return True
      if settings.pydicom_read_force:
          try:
              dicom_headers = pydicom.read_file(filename, defer_size="1 KB", stop_before_pixels=True, force=True)
              if dicom_headers is not None:
                  return True
          except:
              pass
      return False
docs: Util function to check if file is a dicom file
      the first 128 bytes are preamble
      the next 4 bytes should contain DICM otherwise it is not a dicom
  
      :param filename: file to check for the DICM header block
      :type filename: six.string_types
      :returns: True if it is a dicom file