Capybara::ElementNotFound: Unable to find file field "file"

I am testing file upload i.e CSV. In my code as well as browser HTML I found file field but while testing the capybara is unable to find the file field. I tried hard and different approaches but unable to fix the problem. Here partial look like this:

#add_file_box
  %div.msg
  %h1.page-header
    = "Upload a CSV"
  %h4.title

  = form_tag dummy_path, multipart: true, class: "upload_csv" do
    = hidden_field_tag :dmp_id, @dmp.id
    .form-group
      .input-group
        %span.input-group-btn
          %span.btn.btn-primary.btn-file
            Choose file
            = file_field_tag :file, style: 'line-height: normal',  accept: "text/csv", class: "file_input"
        %input.form-control.input-custom{:readonly => "", :type => "text"}
    .form-group
      = submit_tag "Upload CSV", class: "btn btn-primary", id: "upload_csv"

And capybara test look like this

 within '.upload_csv' do
     page.attach_file('file', "#{Rails.root}/spec/acceptance/test_uploads/input_output.csv")
     click_button 'Upload'
   end

I will be thankful if you can help me fixing this problem?

Capybara 2x (capybara issue) doesn't find hidden elements by default.

You can either set ignore_hidden_elements to false:

Capybara.ignore_hidden_elements = false

Or simply add :visible option to your method:

within '.upload_csv' do
  attach_file('file', "#{Rails.root}/spec/acceptance/test_uploads/input_output.csv", visible: false)
    click_button 'Upload'
end

This solved my problem.

Note: :visible option is also supported by most of Capybara methods that internally work with Capybara::Query (like find, all, has_css?, have_selector etc.)