Upload files directly to Amazon S3 from the browser using HTTP POST. Generate policy and signature using an express.js server (node.js).

browser.js

// Requires underscore.js and jquery.js
 
$.ajax({
    type: 'GET',
    url: 'http://your-server.com/signature',
    success: function(response){
        var data = new FormData();
        _.each(response.fields, function(val, key){
            data.append(key, val);
        });
        data.append('file', yourFile);
        $.ajax({
            type: 'POST',
            url: response.url,
            data: data,
            processData: false,
            contentType: false
        });
    },
    dataType: 'json'
});

server.js

var express = require('express'),
    http = require('http'),
    path = require('path'),
    crypto = require('crypto'),
    moment = require('moment'),
    uuid = require('node-uuid');

var app = express();

// Your express config, etc.

var publicKey = 'YOUR_PUBLIC_KEY';
var privateKey = 'YOUR_PRIVATE_KEY';
var bucket = 'YOUR_BUCKET';
var uploadDir = 'uploads/';
var mimetype = 'image/png';
var acl = 'public-read';
var policy = new Buffer(JSON.stringify({
    'expiration': moment().add('minutes', 5).utc().format('YYYY-MM-DDTHH:mm:ss') + 'Z',
    'conditions': [
        { 'bucket': bucket }, 
        [ 'starts-with', '$key', uploadDir ],
        { 'acl': acl },
        [ 'eq', '$Content-Type', mimetype ],
        [ 'content-length-range', 0, 10490000 ], 
    ]
})).toString('base64');
var signature = crypto.createHmac('sha1', privateKey).update(policy).digest('base64');

app.get('/signature', function(req, res){
    res.setHeader('Content-Type', 'application/json');
    res.end(JSON.stringify({
        url: 'https://' + bucket + '.s3.amazonaws.com/',
        fields: {
            key: uploadDir + uuid.v4() + '.png',
            AWSAccessKeyId: publicKey,
            acl: acl,
            'Content-Type': mimetype,
            policy: policy,
            signature: signature
        }
    }));
});