How to send image as base64 string in JSON using HTTP POST?

Question

I'm trying to upload an image from user's computer. First I read the image as a binary string, then encode it in base64 and then POST it as part of a JSON object. However, I get a HTTP 500 error and I have no idea why. I have tried replacing reader.result with a hardcoded short string and it worked (I got a HTTP 200). So I'm not sure if the string length (which is very long for an image binary string) is the major factor here. The domain from which the POST is being sent and received are different if that matters. What could be wrong here? Thanks in advance.

Error message:

[2014-01-11 14:23:58] INFO WEBrick::HTTPServer#start: pid=77263 port=9292 127.0.0.1 - - [11/Jan/2014 14:24:37] "POST /pull_request HTTP/1.1" 500 185316 0.0413

Client side code:

if(image && image.type.match(imageType)) {
    var fileName = timestamp+'-'+image.name;
    var reader = new FileReader();
    reader.onload = function(e) {
        write('images/'+fileName, Base64.encode(reader.result), 'base64');
    };
    reader.readAsBinaryString(image);
}

function write(path, content, encoding){
    var request = {"request":{"path": path, "content": content, "encoding": encoding}};
    $.ajax({
        type: "POST",
        url: "http://localhost:9292/upload",
        data: JSON.stringify(request)
    });

}

Server side code. I removed the JSON parsing and upload stuff for simplicity because at the moment I just want to get a HTTP 200 whenever I receive a POST, regardless of data content:

require 'sinatra'
require 'json/ext' # required for .to_json
require 'github_api'

configure do
    username = ENV['GITHUB_USER'] || 'username'
    password = ENV['GITHUB_PASSWORD'] || 'password'
    github_login = "%s:%s"  % [username, password]
    github = Github.new basic_auth: github_login
    set :github, github
    set :user_name, 'someusername'
    set :repo, 'somerepo'
end

post "/request" do
    status 200
end