You are trying to read a directory... what you need is the file! Do something like this... then, you can read the file as you want.
File dir = Environment.getExternalStorageDirectory();
File yourFile = new File(dir, "path/to/the/file/inside/the/sdcard.ext");
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…