Using Java 7

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class Test {
  public static void main(String[] args) throws IOException {
    Path source = Paths.get("c:/temp/0multipage.tif");
    System.out.println(Files.probeContentType(source));
    // output : image/tiff
  }
}

Using javax.activation.MimetypesFileTypeMap

The MimetypesFileMap class is used to map a File to a Mime Type. Mime types supported are defined in a ressource file inside the activation.jar.

import javax.activation.MimetypesFileTypeMap;
import java.io.File;

class GetMimeType {
  public static void main(String args[]) {
    File f = new File("gumby.gif");
    System.out.println("Mime Type of " + f.getName() + " is " +
                         new MimetypesFileTypeMap().getContentType(f));
    // expected output :
    // "Mime Type of gumby.gif is image/gif"
  }
}

The MimetypesFileTypeMap looks in various places in the user's system for MIME types file entries. When requests are made to search for MIME types in the MimetypesFileTypeMap, it searches MIME types files in the following order:

1. Programmatically added entries to the MimetypesFileTypeMap instance.
2. The file .mime.types in the user's home directory.
3. The file <java.home>/lib/mime.types.
4. The file or resources named META-INF/mime.types.
5. The file or resource named META-INF/mimetypes.default (usually found only in the activation.jar file).

Using java.net.URL

Like the above method a match is done with the extension. The mapping between the extension and the mime-type is defined in the file [jre_home]\lib\content-types.properties

import java.net.*;

public class FileUtils{
  public static String getMimeType(String fileUrl)
    throws java.io.IOException, MalformedURLException
  {
    String type = null;
    URL u = new URL(fileUrl);
    URLConnection uc = null;
    uc = u.openConnection();
    type = uc.getContentType();
    return type;
  }

  public static void main(String args[]) throws Exception {
    System.out.println(FileUtils.getMimeType("file://c:/temp/test.TXT"));
    // output :  text/plain
  }
}

I was trying to find the best way of getting the mime type of a file
and found your sight very useful. However I have now found a way of
getting the mime type using URLConnection that isn't as slow as the
way you describe.

import java.net.FileNameMap;
import java.net.URLConnection;

public class FileUtils {

  public static String getMimeType(String fileUrl)
      throws java.io.IOException
    {
      FileNameMap fileNameMap = URLConnection.getFileNameMap();
      String type = fileNameMap.getContentTypeFor(fileUrl);

      return type;
    }

    public static void main(String args[]) throws Exception {
      System.out.println(FileUtils.getMimeType("file://c:/temp/test.TXT"));
      // output :  text/plain
    }
  }

Using Apache Tika

This package is very up-to-date regarding the filetypes supported, Office 2007 formats are supported (docs/pptx/xlsx/etc...).

Tika has a lot of dependencies ... almost 20 jars ! But it can do a lot more than detecting filetype. For example, you can parse a PDF or DOC to extract the text and the metadata very easily.

import java.io.File;
import java.io.FileInputStream;

import org.apache.tika.metadata.Metadata;
import org.apache.tika.parser.AutoDetectParser;
import org.apache.tika.parser.Parser;
import org.apache.tika.sax.BodyContentHandler;
import org.xml.sax.ContentHandler;

public class Main {

    public static void main(String args[]) throws Exception {

    FileInputStream is = null;
    try {
      File f = new File("C:/Temp/mime/test.docx");
      is = new FileInputStream(f);

      ContentHandler contenthandler = new BodyContentHandler();
      Metadata metadata = new Metadata();
      metadata.set(Metadata.RESOURCE_NAME_KEY, f.getName());
      Parser parser = new AutoDetectParser();
      // OOXMLParser parser = new OOXMLParser();
      parser.parse(is, contenthandler, metadata);
      System.out.println("Mime: " + metadata.get(Metadata.CONTENT_TYPE));
      System.out.println("Title: " + metadata.get(Metadata.TITLE));
      System.out.println("Author: " + metadata.get(Metadata.AUTHOR));
      System.out.println("content: " + contenthandler.toString());
    }
    catch (Exception e) {
      e.printStackTrace();
    }
    finally {
        if (is != null) is.close();
    }
  }
}

Using JMimeMagic

// snippet for JMimeMagic lib
//     http://sourceforge.net/projects/jmimemagic/

Magic parser = new Magic() ;
// getMagicMatch accepts Files or byte[],
// which is nice if you want to test streams
MagicMatch match = parser.getMagicMatch(new File("gumby.gif"));
System.out.println(match.getMimeType()) ;

Using mime-util

import eu.medsea.mimeutil.MimeUtil;

public class Main {
    public static void main(String[] args) {
        MimeUtil.registerMimeDetector("eu.medsea.mimeutil.detector.MagicMimeMimeDetector");
        File f = new File ("c:/temp/mime/test.doc");
        Collection<?> mimeTypes = MimeUtil.getMimeTypes(f);
        System.out.println(mimeTypes);
        //  output : application/msword
    }
}

Using Droid

DROID uses internal and external signatures to identify and report the specific file format versions of digital files. These signatures are stored in an XML signature file, generated from information recorded in the PRONOM technical registry. New and updated signatures are regularly added to PRONOM, and DROID can be configured to automatically download updated signature files from the PRONOM website via web services.

It can be invoked from two interfaces, a Java Swing GUI or a command line interface.

Aperture framework

The Aperture code consists of a number of related but independently usable parts:

• Crawling of information sources: file systems, websites, mail boxes
• MIME type identification
• Full-text and metadata extraction of various file formats
• Opening of crawled resources