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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def subjects_download(self, subject_id):
 """Get data file for subject with given identifier. Parameters subject_id : string Unique subject identifier Returns ------- FileInfo Information about subject's data file on disk or None if identifier is unknown """

        # Retrieve subject to ensure that it exist
        subject = self.subjects_get(subject_id)
        if subject is None:
            # Return None if subject is unknown
            return None
        else:
            # Reference and information for original uploaded file
            return FileInfo(
                subject.data_file,
                subject.properties[datastore.PROPERTY_MIMETYPE],
                subject.properties[datastore.PROPERTY_FILENAME]
            )