If, when reading a DICOM file, a minimum amount of meta information is
not found in the first 4K, then read the entire file.  Some modalities,
like RTRECORD, have more information before basic info like patient ID.
When reading the firt 4K, if no meta information is found, then it is
assumed that this is not a DICOM file and fails immediatly.  Note that
the 4K is actually the constant DICOMClient.DICOM_METADATA_LENGTH.