Reading CSV File without manupulating encoding

I have a csv file that I can see with :

file -bi myCsv.txt

that the character-set is iso-8859-1.Now I want to read this file using JavaScript from input tag and type 'file' without manipulating the encoding.

I tried several options with FileReader() with a method readAsText() and readAsArrayBuffer() or with a fetch API and new Uint8Array(arrayBuffer), but every time I send the file (as string or byte[]) to the backend via ajax-post-request, I see UTF-8 and not iso-8859-1 anymore.

I thought that if I read just a bytes of this file, i would not manipulate the encoding.But with FileReader() and fetch-API(URL) the encoding is changed every time.How can i read text files without changing the encoding to check the encoding in the backend, which works fine.

Here is Update, when i'am sending that as byte[]

async function sendFileToValidation() {      const file = document.getElementById("uploadFile").files[0];      try {        const fileURL = URL.createObjectURL(file);                      const response = await fetch(fileURL); //no idea, which encoding is set        const arrayBuffer = await response.arrayBuffer();        const byteArray = new Uint8Array(arrayBuffer);               $.ajax({          url: '/validateOnBackend',          type: 'POST',          contentType: 'application/octet-stream',          data: byteArray,          success: function (response) {            alert("we can process this encoding!!!!" + response);          },          error: function (xhr, status, error) {            alert("Error: " + xhr.responseText);          }        });        return byteArray;      } catch (error) {        console.error('error', error);      }    }