octet stream header 분석

``` import java.io.File; import java.io.FileInputStream; import java.io.IOException; import lombok.extern.slf4j.Slf4j;

@Slf4j public class OctetStream {

public String changeFileNameForExtension(File originFile) throws IOException { byte[] buf = new byte[32]; try (FileInputStream fis = new FileInputStream(originFile)) {

  fis.read(buf);

} catch (Exception e) {
  throw new IOException(" file read Error : " + originFile);
}

String extractedExt = getExtFromFileHeaderFormat(buf);

if (extractedExt == null) {
  throw new IOException(
      "Unknown Image format found.. Cannot infer file type! " + originFile.getName());
}

log.info("Infer success : file format was {} so start renaming", extractedExt);

String onlyFileNameWithoutExtension = com.google.common.io.Files.getNameWithoutExtension(originFile.getName());
File renamedFile = new File(originFile.getParent() + "/" + onlyFileNameWithoutExtension + "." + extractedExt);
if (originFile.renameTo(renamedFile)) {
  log.debug("new file name from {} to {}", originFile.getName(), renamedFile.getName());
}
return renamedFile.getName();   }

private String getExtFromFileHeaderFormat(byte[] targets) {

final byte[][][] checksum = new byte[][][]{
    {
        /* GIF CASE 1*/
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0xFF},
        {(byte) 0x47, (byte) 0x49, (byte) 0x46, (byte) 0x38, (byte) 0x00, (byte) 0x61}
    },
    {
        /* GIF CASE 2*/
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0xFF},
        {(byte) 0x47, (byte) 0x49, (byte) 0x46, (byte) 0x38, (byte) 0x00, (byte) 0x61}
    },
    {
        /* PNG CASE */
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF,
            (byte) 0xFF, (byte) 0xFF},
        {(byte) 0x89, (byte) 0x50, (byte) 0x4e, (byte) 0x47, (byte) 0x0d, (byte) 0x0a,
            (byte) 0x1a, (byte) 0x0a}
    },
    {
        /* JPG CASE 1 */
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0x00,
            (byte) 0xFF, (byte) 0xFF},
        {(byte) 0xFF, (byte) 0xD8, (byte) 0xFF, (byte) 0xE0, (byte) 0x00, (byte) 0x00,
            (byte) 0x4A, (byte) 0x46}
    },
    {
        /* JPG CASE 2 */
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0x00,
            (byte) 0xFF, (byte) 0xFF},
        {(byte) 0xFF, (byte) 0xD8, (byte) 0xFF, (byte) 0xE1, (byte) 0x00, (byte) 0x00,
            (byte) 0x45, (byte) 0x78}
    },
    {
        /* JPG CASE 3 */
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0x00,
            (byte) 0xFF, (byte) 0xFF},
        {(byte) 0xFF, (byte) 0xD8, (byte) 0xFF, (byte) 0xE8, (byte) 0x00, (byte) 0x00,
            (byte) 0x53, (byte) 0x50}
    },
    {
        /* WEBP CASE 3 */
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0x00,
            (byte) 0x00, (byte) 0x00},
        {(byte) 0x52, (byte) 0x49, (byte) 0x46, (byte) 0x46, (byte) 0x00, (byte) 0x00,
            (byte) 0x00, (byte) 0x00},
        {(byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0x00, (byte) 0x00,
            (byte) 0x00, (byte) 0x00},
        {(byte) 0x57, (byte) 0x45, (byte) 0x42, (byte) 0x50, (byte) 0x00, (byte) 0x00,
            (byte) 0x00, (byte) 0x00}
    }
};

final String[] returnValues = new String[]{
    "GIF", "GIF", "PNG", "JPG", "JPG", "JPG", "WEBP"
};

boolean isAllZero = false;

for (int i = 0; i mime-type octet stream일 경우 파일 헤더 분석을 통해 확장자 매칭 코드