How to get a file's Media Type (MIME type)?

In Java 7 you can now just use Files.probeContentType(path).

Unfortunately,

mimeType = file.toURL().openConnection().getContentType();

does not work, since this use of URL leaves a file locked, so that, for example, it is undeletable.

However, you have this:

mimeType= URLConnection.guessContentTypeFromName(file.getName());

and also the following, which has the advantage of going beyond mere use of file extension, and takes a peek at content

InputStream is = new BufferedInputStream(new FileInputStream(file));
mimeType = URLConnection.guessContentTypeFromStream(is);
 //...close stream

However, as suggested by the comment above, the built-in table of mime-types is quite limited, not including, for example, MSWord and PDF. So, if you want to generalize, you'll need to go beyond the built-in libraries, using, e.g., Mime-Util (which is a great library, using both file extension and content).

With Apache Tika you need only three lines of code:

File file = new File("/path/to/file");
Tika tika = new Tika();
System.out.println(tika.detect(file));

If you have a groovy console, just paste and run this code to play with it:

@Grab('org.apache.tika:tika-core:1.14')
import org.apache.tika.Tika;

def tika = new Tika()
def file = new File("/path/to/file")
println tika.detect(file)

Keep in mind that its APIs are rich, it can parse "anything". As of tika-core 1.14, you have:

String  detect(byte[] prefix)
String  detect(byte[] prefix, String name)
String  detect(File file)
String  detect(InputStream stream)
String  detect(InputStream stream, Metadata metadata)
String  detect(InputStream stream, String name)
String  detect(Path path)
String  detect(String name)
String  detect(URL url)

See the apidocs for more information.