Something like this is what I use all the time. No need for any base64 decoding.
<html>
<head>
<script>
window.onload = function(event) {
document.getElementById('fileInput').addEventListener('change', handleFileSelect, false);
}
function handleFileSelect(event) {
var fileReader = new FileReader();
fileReader.onload = function(event) {
$('#accessKeyField').val(event.target.result);
}
var file = event.target.files[0];
fileReader.readAsText(file);
document.getElementById('fileInput').value = null;
}
</script>
</head>
<body>
<input type="file" id="fileInput" style="height: 20px; width: 100px;">
</body>
</html>