通过JavaScript上传文件：服务器上的文件无效

Question

I'm trying to upload a binary file (a PDF to be specific) with this JS-Snippet:

function uploadFile() {
    var reader = new FileReader();
    var file = document.getElementById('uploadInput').files[0];
    console.log(file.size);
    var xhr = new XMLHttpRequest();

    xhr.open('POST', 'custom?id=upload');
    xhr.setRequestHeader("Content-Type", "application/pdf");
    xhr.overrideMimeType('application/pdf');
    reader.onload = function(evt) {
        xhr.send(evt.target.result);
    };
    reader.readAsBinaryString(file);
}

On the serverside (Java) I'm receiving the request and writing the file to disk.
But instead of the expected ~3MB I'm getting ~4MB, with the effect that I only have blank pages, when opening the PDF.

The header of the incoming request also specifies a Content-Length of ~4MB, so I'm fairly certain, that it is something on the clientside, that is causing the trouble.
Sending plain text files is no problem at all, they arrive exactly as they are supposed to be.

Since writing JavaScript and working in the web is not in my daily field of work, it is very well possible that I did something fundamentelly wrong.

Answer 1

I would suspect 2 things you can try out:

• check the MIME type on both end of the process, if it gets mangled somehow you have your answer
• Your upload gets wrapped into a XMLHttprequest - you might have to Base64 Encode/Decode manually to get rid of the native byte representation in the PDF

Hopefully this will help you, good luck!

Answer 2

I found the solution: encode the data explicitly with Base64 before sending them and decode them appropriately on the receiving end.

function uploadFile() {
    var reader = new FileReader();
    var file = document.getElementById('uploadInput').files[0];
    console.log(file.size);
    var xhr = new XMLHttpRequest();

    xhr.open('POST', 'custom?id=upload');
    xhr.setRequestHeader("Content-Type", "text/plain; charset=x-user-defined");
    xhr.overrideMimeType('text/plain; charset=x-user-defined');
    reader.onload = function(evt) {
        var encoded = window.btoa(evt.target.result);
        xhr.send(encoded);
    };
    reader.readAsBinaryString(file);
}

I'm writing this to let anyone, who got the same problem, know of my solution.