Using Amazon S3 as a content addressable store.

cors.xml

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
    <CORSRule>
        <AllowedOrigin>*</AllowedOrigin>
        <AllowedMethod>GET</AllowedMethod>
        <MaxAgeSeconds>3000</MaxAgeSeconds>
        <AllowedHeader>Content-*</AllowedHeader>
    </CORSRule>
</CORSConfiguration>

s3cas.rb

require "digest/sha1"
require "fog"
require "sinatra"
require "tempfile"

configure do
  storage = Fog::Storage.new({
    :provider => 'AWS',
    :aws_access_key_id => ENV["ACCESS_KEY_ID"],
    :aws_secret_access_key => ENV["SECRET_ACCESS_KEY"],
  })

  set :bucket, ENV["AWS_BUCKET"]

  set :storage, storage
end

post "/upload" do
  if data = params[:data]
    if data.is_a? String
      content_type = params[:type]
      file = Tempfile.new ["data", ".json"]
      file.write data
      file.rewind
    else
      file = data[:tempfile]
      content_type = data[:type]
    end
  elsif data = params[:data_base64]
    content_type = params[:type]
    file = Tempfile.new ["image", ".png"], :encoding => 'ascii-8bit'
    file.write Base64.decode64(data)
    file.rewind
  else
    return 400
  end

  store(file, content_type)

  200
end

def directory
  settings.storage.directories.get(settings.bucket)
end

def store(file, content_type=nil)
  sha1 = Digest::SHA1.file(file.path).hexdigest

  if directory.files.get(sha1)
    # Already stored, do nothing
  else
    directory.files.create(
      :key => sha1,
      :body => file,
      :content_type => content_type,
      :public => true,
      :metadata => {
        'Cache-Control' => 'max-age=315576000'
      }
    )
  end
end