Example file type detector

src/test/java/com/github/fge/filesystem/ftd/PngFileTypeDetector.java

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+package com.github.fge.filesystem.ftd;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.nio.file.spi.FileTypeDetector;
+import java.util.Arrays;
+
+public final class PngFileTypeDetector
+    extends FileTypeDetector
+{
+    private static final byte[] PNG_HEADER = {
+        (byte) 0x89,
+        (byte) 0x50, (byte) 0x4E, (byte) 0x47,
+        (byte) 0x0D, (byte) 0x0A,
+        (byte) 0x1A,
+        (byte) 0x0A
+    };
+
+    private static final int PNG_HEADER_SIZE = PNG_HEADER.length;
+
+    @Override
+    public String probeContentType(final Path path)
+        throws IOException
+    {
+        final byte[] buf = new byte[PNG_HEADER_SIZE];
+
+        try (
+            final InputStream in = Files.newInputStream(path);
+        ) {
+            if (in.read(buf) != PNG_HEADER_SIZE)
+                return null;
+        }
+
+        return Arrays.equals(buf, PNG_HEADER) ? "image/png" : null;
+    }
+}

src/test/resources/META-INF/services/java.nio.file.spi.FileTypeDetector

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+com.github.fge.filesystem.ftd.PngFileTypeDetector