Erik A. Hanson's Weblog

I wrote a web app on top of Rack that includes the ability to accept file uploads. It took some time to figure out how to successfully test the file upload using Rack::Test.

The trick was to encode the file exactly correctly, and then post it with the correct headers. I wrote a simple post_file method to do this.

  def post_file(url, param_name, file_path, content_type)
    file_name = File.basename(file_path)
    boundary = "AaB03x"
    data = "--#{boundary}\r\n" +
           "Content-Disposition: form-data; name=\"#{param_name}\"; filename=\"#{file_name}\"\r\n" +
           "Content-Type: #{content_type}\r\n" +
           "\r\n" +
           "#{File.read(file_path)}\r\n" +
           "--#{boundary}--\r\n" +
           "\r\n"
    post(url, {}, {
      "CONTENT_TYPE" => "multipart/form-data, boundary=\"#{boundary}\"",
      "CONTENT_LENGTH" => data.length, :input => data })
  end

I call it from my test like this:

...
post_file("/my_app/upload", "uploaded_image", "../test-images/originals/loki.jpg", "image/jpeg")
...

and my app reads it like this:

...
request = Rack::Request.new(env)
...
uploaded_image = request.params["uploaded_image"][:tempfile]
...