How do you test uploading a file with capybara and Dropzone?

I've switched to using the Dropzone.js plugin for drag-and-drop file uploads. How can I write a Capybara test to ensure this functionality keeps on working?

Previously I had a template with an input file element:

<input type="file" name="attachments">

And the test was simple:

When(/^I upload "([^"]*)"$/) do |filename|
  attach_file("attachments", File.expand_path(filename))
  # add assertion here
end

However this no longer works because Dropzone doesn't have a visible file input.