The specification of URLConnection.getContentType() says:

    Returns the value of the content-type header field.

Returns:
    the content type of the resource that the URL references, or null if not known.

However, if the URL is a file URL pointing to a plain file,
URLConnection.getContentType() returns "content/unknown" but content-type
header field is null.

import java.net.*;

public class ContentTypeTest {
    public static void main(String[] args) throws Exception {
        URL url = new URL("file:///etc/passwd");
        URLConnection conn = url.openConnection();
        conn.connect();
        System.out.println(conn.getContentType());
        System.out.println(conn.getHeaderField("content-type"));
    }
}