将文件上传转换为二进制文件，发布到服务器并使用PHP保存文件

I am working with an app where I am unable to post the File to the server. Therefor I have chose to send it as a string to the server, and remake it to a file using PHP. Below is the javascript code I am using to convert the image to a string.

var file = document.getElementById("fileForUpload").files[0];
if (file) {
    var reader = new FileReader();
    reader.readAsText(file);
    reader.onload = function (evt) {
        document.getElementById("fileContents").value = utf8_to_b64(evt.target.result);
    }
    reader.onerror = function (evt) {
        document.getElementById("fileContents").value = "error reading file";
    }
}

function utf8_to_b64(str) {
    return window.btoa(unescape(encodeURIComponent(str)));
}

On the server side I'm doing this

header("Content-type: image/png");
$data = preg_replace('/\s+/', '', $data);
echo base64_decode($data);
exit;

But it says that the image cannot be displayed because it contains errors.

Can you please tell me what I am doing wrong here? I am correctly receiving the Base64 encoded string to the server.

Edit

Please note that I am trying to post the string through an HTML form.

Easy:

$imagedata = file_get_contents("/path/to/image.jpg");
             // alternatively specify an URL, if PHP settings allow
$base64 = base64_encode($imagedata);
bear in mind that this will enlarge the data by 33%, and you'll have problems with files whose size exceed your memory_limit.