Add getting Extension from MIME type of a file

core-java-modules/core-java-io/pom.xml

@@ -43,6 +43,17 @@
             <version>${angus-activation.version}</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.jodd</groupId>
+            <artifactId>jodd-util</artifactId>
+            <version>6.2.1</version>
+        </dependency>
+        <dependency>
+            <groupId>com.j256.simplemagic</groupId>
+            <artifactId>simplemagic</artifactId>
+            <version>1.17</version>
+        </dependency>
+
     </dependencies>
 
     <build>

core-java-modules/core-java-io/src/test/java/com/baeldung/extension/ExtensionFromMimeTypeUnitTest.java

@@ -0,0 +1,47 @@
+package com.baeldung.extension;
+
+import static org.junit.Assert.assertEquals;
+import java.util.HashMap;
+import java.util.Map;
+
+import org.apache.tika.mime.MimeType;
+import org.apache.tika.mime.MimeTypeException;
+import org.apache.tika.mime.MimeTypes;
+import org.junit.Test;
+
+import com.j256.simplemagic.ContentInfo;
+
+public class ExtensionFromMimeTypeUnitTest {
+    public static final String PNG_EXT = "image/png";
+    @Test
+    public void whenUsingTika_thenGetFileExtension() throws MimeTypeException {
+        MimeTypes allTypes = MimeTypes.getDefaultMimeTypes();
+        MimeType type = allTypes.forName(PNG_EXT);
+        String extension = type.getExtension();
+        assertEquals(".png", extension);
+    }
+
+    @Test
+    public void  whenUsingJodd_thenGetFileExtension() {
+        String fileExtension = jodd.net.MimeTypes.findExtensionsByMimeTypes(PNG_EXT,false)[0];
+        assertEquals("png", fileExtension);
+    }
+
+    @Test
+    public void whenUsingMimetypesFileTypeMap_thenGetFileExtension() {
+        ContentInfo contentInfo = new ContentInfo("", PNG_EXT, "", true);
+        String[] fileExtensions = contentInfo.getFileExtensions();
+        assertEquals("png", fileExtensions[0]);
+    }
+
+    @Test
+    public void whenUsingCustomLogic_thenGetFileExtension() {
+        Map<String, String> mimeToExtensionMap = new HashMap<>();
+        mimeToExtensionMap.put("application/pdf", "pdf");
+        mimeToExtensionMap.put("image/jpeg", "jpg");
+        mimeToExtensionMap.put("image/png", "png");
+        String fileExtension = mimeToExtensionMap.get(PNG_EXT);
+        if (fileExtension!= null)
+            assertEquals("png", fileExtension);
+    }
+}