MediaTypeDetector.java

import static org.springframework.http.MediaType.*;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Path;
import java.util.LinkedList;
import java.util.List;
import org.springframework.http.MediaType;
import org.springframework.stereotype.Component;

import static java.lang.System.arraycopy;

@Component
public class MediaTypeDetector {

	private static final int SIGNATURE_MAX_LENGTH = 11;
	private static final int END_OF_SIGNATURE = -2;

	private static final String APP = "application";
	private static final String IMAGE = "image";
	private static final String AUDIO = "audio";

	public static final MediaType APP_EPS = new MediaType(APP, "eps");
	public static final MediaType APP_JAVA = new MediaType(APP, "java");
	public static final MediaType APP_JAVA_OBJECT = new MediaType(APP, "x-java-serialized-object");
	public static final MediaType APP_PS = new MediaType(APP, "postscript");
	public static final MediaType APP_ZIP = new MediaType(APP, "zip");
	public static final MediaType AUDIO_BASIC = new MediaType(AUDIO, "basic");
	public static final MediaType AUDIO_MP3 = new MediaType(AUDIO, "mp3");
	public static final MediaType AUDIO_WAV = new MediaType(AUDIO, "x-wav");
	public static final MediaType IMAGE_BMP = new MediaType(IMAGE, "bmp");
	public static final MediaType IMAGE_SVG = new MediaType(IMAGE, "svg+xml");
	public static final MediaType IMAGE_TIFF = new MediaType(IMAGE, "tiff");

	private static final List<Signature> Signatures = new LinkedList<>();

	static {
		Signatures.add(Signature.of(IMAGE_SVG, 0x3C, 0x73, 0x76, 0x67, 0x20));	// "<svg "
		Signatures.add(Signature.of(IMAGE_PNG, 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A));
		Signatures.add(Signature.of(IMAGE_JPEG, 0xFF, 0xD8, 0xFF, 0xE0));
		Signatures.add(Signature.of(IMAGE_JPEG, 0xFF, 0xD8, 0xFF, 0xEE));
		Signatures.add(Signature.of(IMAGE_JPEG, 0xFF, 0xD8, 0xFF, 0xE1, -1, -1, 0x45, 0x78, 0x69, 0x66, 0x00));
		Signatures.add(Signature.of(IMAGE_TIFF, 0x49, 0x49, 0x2A, 0x00));
		Signatures.add(Signature.of(IMAGE_TIFF, 0x4D, 0x4D, 0x00, 0x2A));
		Signatures.add(Signature.of(IMAGE_GIF, 0x47, 0x49, 0x46, 0x38));
		Signatures.add(Signature.of(APPLICATION_PDF, 0x25, 0x50, 0x44, 0x46, 0x2D, 0x31, 0x2E));
		Signatures.add(Signature.of(APP_EPS, 0x25, 0x21, 0x50, 0x53, 0x2D, 0x41, 0x64, 0x6F, 0x62, 0x65, 0x2D));
		Signatures.add(Signature.of(APP_PS, 0x25, 0x21, 0x50, 0x53));
		Signatures.add(Signature.of(AUDIO_MP3, 0xFF, 0xFB, 0x30));
		Signatures.add(Signature.of(AUDIO_MP3, 0x49, 0x44, 0x33));
		Signatures.add(Signature.of(TEXT_HTML, 0x3C, 0x21));
		Signatures.add(Signature.of(TEXT_HTML, 0x3C, 0x68, 0x74, 0x6D, 0x6C));
		Signatures.add(Signature.of(TEXT_HTML, 0x3C, 0x68, 0x65, 0x61, 0x64));
		Signatures.add(Signature.of(TEXT_HTML, 0x3C, 0x62, 0x6F, 0x64, 0x79));
		Signatures.add(Signature.of(TEXT_HTML, 0x3C, 0x48, 0x54, 0x4D, 0x4C));
		Signatures.add(Signature.of(TEXT_HTML, 0x3C, 0x48, 0x45, 0x41, 0x44));
		Signatures.add(Signature.of(TEXT_HTML, 0x3C, 0x42, 0x4F, 0x44, 0x59));
		Signatures.add(Signature.of(TEXT_XML, 0x3C, 0x3F, 0x78, 0x6D, 0x6C, 0x20));
		Signatures.add(Signature.of(TEXT_XML, 0xFE, 0xFF, 0x00, 0x3C, 0x00, 0x3f, 0x00, 0x78));
		Signatures.add(Signature.of(TEXT_XML, 0xFF, 0xFE, 0x3C, 0x00, 0x3F, 0x00, 0x78, 0x00));
		Signatures.add(Signature.of(IMAGE_BMP, 0x42, 0x4D));
		Signatures.add(Signature.of(AUDIO_BASIC, 0x2E, 0x73, 0x6E, 0x64));
		Signatures.add(Signature.of(AUDIO_BASIC, 0x64, 0x6E, 0x73, 0x2E));
		Signatures.add(Signature.of(AUDIO_WAV, 0x52, 0x49, 0x46, 0x46));
		Signatures.add(Signature.of(APP_ZIP, 0x50, 0x4B));
		Signatures.add(Signature.of(APP_JAVA, 0xCA, 0xFE, 0xBA, 0xBE));
		Signatures.add(Signature.of(APP_JAVA_OBJECT, 0xAC, 0xED));
	}

	private static class Signature {
		int[] magicBytes;
		MediaType mediaType;

		static Signature of(MediaType type, int... data) {
			Signature result = new Signature();
			result.mediaType = type;

			if (data.length > SIGNATURE_MAX_LENGTH) {
				throw new IllegalArgumentException();
			}

			result.magicBytes = new int[SIGNATURE_MAX_LENGTH];
			int i = -1;
			while (++i < data.length) {
				result.magicBytes[i] = data[i];
			}

			while (i < SIGNATURE_MAX_LENGTH) {
				result.magicBytes[i++] = END_OF_SIGNATURE;
			}

			return result;
		}

		boolean matches(int[] source) {
			int i = -1;
			boolean matches = true;

			while (++i < SIGNATURE_MAX_LENGTH && magicBytes[i] != END_OF_SIGNATURE && matches) {
				matches = magicBytes[i] == source[i] || magicBytes[i] == -1;
			}

			return matches;
		}
	}

	public MediaType getMediaType(final Path path) throws IOException {
		return getMediaType(path.toFile());
	}

	public MediaType getMediaType(final java.io.File file) throws IOException {
		try (final InputStream fis = new FileInputStream(file)) {
			return getMediaType(fis);
		}
	}

	public MediaType getMediaType(final InputStream is) throws IOException {
		final byte[] input = new byte[SIGNATURE_MAX_LENGTH];
		final int count = is.read(input, 0, SIGNATURE_MAX_LENGTH);

		if (count > 1) {
			final byte[] available = new byte[count];
			arraycopy(input, 0, available, 0, count);
			return getMediaType(available);
		}

		return null;
	}

	public MediaType getMediaType(final byte[] data) {
		if (data == null) {
			throw new IllegalArgumentException("data can not be null");
		}

		final int[] source = { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
		for (int i = 0; i < source.length; i++) {
			source[i] = data[i] & 0xFF;
		}

		for (Signature signature : Signatures) {
			if (signature.matches(source)) {
				return signature.mediaType;
			}
		}

		return null; // No matching signature
	}
}

Derived from https://stackoverflow.com/a/66944463/639520

@erizzo the source citation needs to be added to the source code, not as a comment in GitHub, so that if people copy the code, the citation will be carried forward.