code: private static @Nonnull File getFingerprintFile(@Nonnull byte[] md5sum) {
          assert md5sum.length==16;
          return new File( Jenkins.getInstance().getRootDir(),
              "fingerprints/"+ Util.toHexString(md5sum,0,1)+'/'+Util.toHexString(md5sum,1,1)+'/'+Util.toHexString(md5sum,2,md5sum.length-2)+".xml");
      }
docs: Determines the file name from md5sum.