I finally got it working. This way only a few apps are shown by the chooser (Google Drive, Dropbox, Root Explorer, and Solid Explorer). It's working fine with the two explorers but not with Google Drive and Dropbox (I guess because they cannot access the external storage). The other MIME type like "*/*"
is also possible.
public void openFolder(){
Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
Uri uri = Uri.parse(Environment.getExternalStorageDirectory().getPath()
+ File.separator + "myFolder" + File.separator);
intent.setDataAndType(uri, "text/csv");
startActivity(Intent.createChooser(intent, "Open folder"));
}