Android-accessing file from sdcard?

whole day i was trying to get access of a file “t.txt” that i had put in my sdcard:

FString InPath="/mnt/sdcard/t.txt";
FString InPath="/sdcard/t.txt";
FString InPath="./t.txt";

FPaths::FileExists(InPath);

then i tried to check its existence with FPaths::FileExists(InPath).
but it always was returning false.

what InPath value I should have use to get proper address?

note:
i also have added read permission under:
project settings->android->extra permissions:
android.permission.WRITE_EXTERNAL_STORAGE
android.permission.READ_EXTERNAL_STORAGE