NodeJS缓冲区Base64文件限制

Question

尝试使用Base64上传图像，但如果字符串很长，则存储图像的底部为空白。

upload.js

var uuid = require('node-uuid');
var fs = require('fs');

exports.uploadImage = function(base64Str, callback) {
    var filename = uuid.v1()+'.png';
    var bitmap = new Buffer(base64Str, 'base64');
    fs.writeFileSync(filename, bitmap);
    callback(filename);
};

server.js

var fs = require('fs');
var restify = require('restify');  
var server = restify.createServer();
server.use(restify.bodyParser());

var upload = require('./modules/upload');

server.post('/images', function(req, res) {
    upload.uploadImage(req.params.myImage, function(filename) {
        console.log('processing image');
        res.send(filename);
        res.end();
    });
});

server.listen(3000);

它适用于长度为499016字节的字符串，但如果字符串为847508字节则不起作用。是否有文档大小限制，如果没有，我如何上传和取消编码更长的字符串？