code: public String getMimeType() {
          if (format >= 0 && format < MIME_TYPE_STRINGS.length) {
              if (format == FORMAT_JPEG && progressive) {
                  return "image/pjpeg";
              }
              return MIME_TYPE_STRINGS[format];
          }
          return null;
      }
docs: If {@link #check()} was successful, returns a String with the MIME type of the format.
  
  @return MIME type, e.g. <code>image/jpeg</code>