UpYun 表单上传验证信息 helper 方法，用于生成 input tag

_form.html.erb

<form action="http://v0.api.upyun.com/rbtest/" method="post" enctype="multipart/form-data">
  <%= upyun_form_input_tag(:return_url => new_post_path) %>
  <input type="file" name="file" />
  <button type="submit">上传图片</button>
</form>

upyun_form_upload_input_tag.rb

def upyun_form_input_tag(opts = {})
  form_api_key = "THE API KEY"
  opts[:prefix] ||= ""
  opts[:allow_exts] ||= "jpg,jpeg,gif,png"
  opts[:min_length] ||= 0
  opts[:max_length] ||= 200.kilobyte
  opts[:return_url] ||= root_url
  opts[:notify_url] ||= root_url
  opts[:unsharp] ||= false
  opts[:quality] ||= 95
  hash = {
    "bucket" => "rbtest",
    "expiration" => 20.minutes.since.to_i,
    "save-key" => [opts[:prefix],"{year}/{mon}/{filemd5}{.suffix}"].join("/"),
    "allow-file-type" => opts[:allow_exts],
    "content-length-range" => "#{opts[:min_length].to_i},#{opts[:max_length].to_i}",
    "return-url" => opts[:return_url],
    "notify-url" => opts[:notify_url],
    "x-gmkerl-unsharp" => opts[:unsharp],
    "x-gmkerl-quality" => opts[:quality].to_i
  }
  puts "JSON: #{hash.to_json}"
  policy = Base64.encode64(hash.to_json).gsub("\n","")
  sign = Digest::MD5.hexdigest([policy,form_api_key].join("&"))
  raw [tag(:input, :type => "hidden", :name => "policy", :value => policy),tag(:input, :type => :hidden, :name => "signature",:value => sign)].join("\n")
end