Can't fit file encoding when working with Chrome File System API

I need to read a file which contains a group of symbols moved 65 in ASCII table. It means, for each symbol I am meant to do:

String.fromCharCode('¢'.charCodeAt(0)-65) // returns 'a'

But it is not working at all. I have asked friends of mine to do the test using Python inputting the same file and they got the correct result.

When I try to do the same work with Chrome File System it does not work at all. I can't get back the expected symbols. I think it is a problem with my encoding/charset plataform but I can't figure out what is and how fix it.

I have tried opening the file with other encoding:

var reader=new FileReader();

reader.readAsText(file, 'windows-1252'); // no success
reader.readAsText(file, 'ISO-8859-2'); // no success

Appreciate any help