File upload through API(conversion of base 64)

HTML CODE

<form>
        <label for="submit_sign">Signature:</label>
        <input type="file" id="submit_sign" name="submit_sign" size="50" accept=".doc, .docx, .pdf, .txt, .jpg, .png" value="Upload" onchange="myFunction('submit_sign');" required>
        <input type="hidden" id="submit_sign-code" name="submit_sign-code">
</form>

JS Script

<script>
    function myFunction(id) {
        var files = $('#' + id)[0].files;
        var encodedFile = fileReaders(files[0], id);
    }
</script>

<script type="text/javascript">
   function fileReaders(file, id){
      return new Promise((resolve, reject) => {
        var fr = new FileReader();
        fr.readAsDataURL(file);  
            fr.onload = () => {
                var mtlFileContent = (fr.result).replace('data:;base64,', '');
                var b64 = mtlFileContent.split("base64,")[1];
                $('#'+ id + '-code').val(b64);
            };
        });
    }
</script>