How to test Dropzone.js upload with Rails, Cucumber and Capybara?

I have a Rails project using Cucumber and Capybara for tests. I have a file upload page using Dropzone.js.

My uploads work great using the dialog box or drag and drop. Testing is another matter.

I have the following field in my form:

<input id="photo_image" multiple="multiple" name="image" type="hidden">

However, in the step definitions, I've tried a few methods of finding and attaching the file data, but none of them work.

I've tried fill_in:

fill_in "photo_image",  with: photo

I've tried find with css selectors:

find('#photo_image').set photo

I've tried find with xpath:

find(:xpath, "//input[@id='photo_image']").set photo

But none of them see the hidden field.

Unable to find css "#photo_image" (Capybara::ElementNotFound)

Unable to find xpath "//input[@id='photo_image']" (Capybara::ElementNotFound)

Unable to find field "photo_image" (Capybara::ElementNotFound)

Is there any testing method that can handle the upload using Dropzone.js or is it hopeless?