The problem here is that the gmail app only want to send files which are located on the sdcard
Intent mail = new Intent(android.content.Intent.ACTION_SEND);
mail.setType("application/octet-stream");
mail.putExtra(android.content.Intent.EXTRA_EMAIL, new String[] {"my@mail.com"});
mail.putExtra(android.content.Intent.EXTRA_SUBJECT, "Subject");
mail.putExtra(android.content.Intent.EXTRA_TEXT, "Message");
mail.putExtra(android.content.Intent.EXTRA_STREAM, Uri.parse("file://" + Environment.getExternalStorageDirectory().getAbsolutePath() + "/file.txt"));
PrefAct.startActivity(Intent.createChooser(mail, "Send mail via..."));
As said gmail will refuse your attachment when the user sends the mail when the file isn't located on the ExternalStorage.