由于安卓更新7.0以后对各种权限进行了控制，所以原方法不能使用
如使用：Environment.getExternalStorageDirectory() 会报open failed: EPERM (Operation not permitted)的错误
7.0以后使用：
ContextWrapper cw = new ContextWrapper(getApplicationContext());
File directory = cw.getExternalFilesDir(Environment.DIRECTORY_MUSIC);
File file = new File(directory, "something" + ".MP3");
这里的
Environment.DIRECTORY_MUSIC还有其他形式：
public static String DIRECTORY_MUSIC = "Music";
public static String DIRECTORY_ALARMS = "Alarms";
public static String DIRECTORY_NOTIFICATIONS = "Notifications";
public static String DIRECTORY_PICTURES = "Pictures";
public static String DIRECTORY_MOVIES = "Movies";
public static String DIRECTORY_DOWNLOADS = "Download";
public static String DIRECTORY_DCIM = "DCIM";
public static String DIRECTORY_DOCUMENTS = "Documents";