Worked for me Try this code
uri = Uri.fromFile(new File("//assets/testdemo.txt"));
String testfilepath = uri.getPath();
File f = new File(testfilepath);
if (f.exists() == true) {
Toast.makeText(getApplicationContext(),"valid :" + testfilepath, 2000).show();
} else {
Toast.makeText(getApplicationContext(),"invalid :" + testfilepath, 2000).show();
}