How to remove default input class type from Simple

I'm using Rails 4.2.4 with Bootstrap 4 (using the bootstrap_ruby gem).

Simple Form adds input type classes to the form... if the input is a string it will add a class string to the input. I wanted to know how to stop this from happening?

For example, if I had a form with a file input.

simple_form_for @attachment do |f|
  f.input_field :file, as: :file
end

It will produce the following HTML:

<form>
  ...

  <div class="form-group file optional photo_image">
    <label class="file optional control-label" for="attachment_file">File</label>
    <input class="file optional" type="file" name="attachment[file]" id="attachment_file"></div>

  ...

</form>

It adds the file class to the label and input fields. Is there a way to remove the file class when the form is being built?

I'm trying to use Bootstrap 4 (Alpha) and it's clashing with the file class name.

I thought I could do it on the config.wrappers but it adds the type of input as a class eg. string, file.

Thanks in advance.

Bootstrap 4's naming choice on the custom file field is unfortunate, as it is very generic. Unfortunately, there is no easy way to toggle that automatically added css-classes with SimpleForm.

My solution is to introduce a new input field which just inherits from the FileInput, so it receives a different name and so a different css-class:

// initializer
# create a new file_hack input type same as file

class SimpleForm::Inputs::FileHackInput < SimpleForm::Inputs::FileInput
end

# optional: map file_hack to a input type configuration for easier classes etc.
SimpleForm.setup do |config|
  ...
  config.wrapper_mappings = {
     # check_boxes: :vertical_radio_and_checkboxes,
     # radio_buttons: :horizontal_radio_and_checkboxes,
     # file: :vertical_file_input,
     file_hack: :vertical_file_input,
     # boolean: :vertical_boolean
  }

  config.wrappers :vertical_file_input, tag: 'fieldset', class: 'form-group', error_class: 'has-error' do |b|
    b.use :html5
    b.use :placeholder
    b.use :label, class: 'control-label'

    b.wrapper tag: 'div' do |ba|
      ba.use :input, class: 'form-control-file'
      ba.use :error, wrap_with: { tag: 'span', class: 'help-block' }
      ba.use :hint,  wrap_with: { tag: 'div', class: 'text-muted' }
    end
  end

Call the file field as the new input "type".