title: getMimeTypeFromExtension returns null when I pass "json" as extension
body: <p>The following code returns null</p>
  
  <pre><code>MimeTypeMap.getSingleton().getMimeTypeFromExtension("json");
  </code></pre>
  
  <p>I've tried it with other formats like mp4, png, etc... and it works fine but fails to work for json.</p>
  
  <p>I've also tried <code>URLConnection.guessContentTypeFromName</code> and got null as well. </p>
  
  <p>So how can I get the mime type of a json which I'm expecting it to be <code>"application/json"</code>?</p>
  
  <p><strong>Temporary Solution</strong></p>
  
  <pre><code>public static final String MIME_TYPE_JSON = "application/json";
  
  private static final String EXTENSION_JSON = "json";
  
  @Nullable
  public static String getMimeType(final String path) {
      // StringUtil is my own util but you can use Guava for the same result
      String extension = StringUtil.getExtension(path).toLowerCase();
      return extension.equals(EXTENSION_JSON) ?
                  MIME_TYPE_JSON : MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension);
  }
  </code></pre>