title: How to remove default input class type from Simple Form, using Rails 4 and Bootstrap 4?
body: <p>I'm using Rails 4.2.4 with Bootstrap 4 (using the bootstrap_ruby gem).</p>
  
  <p>Simple Form adds input type classes to the form... if the input is a string it will add a class <code>string</code> to the input. I wanted to know how to stop this from happening?</p>
  
  <p>For example, if I had a form with a file input.</p>
  
  <pre><code>simple_form_for @attachment do |f|
    f.input_field :file, as: :file
  end
  </code></pre>
  
  <p>It will produce the following HTML:</p>
  
  <pre><code>&lt;form&gt;
    ...
  
    &lt;div class="form-group file optional photo_image"&gt;
      &lt;label class="file optional control-label" for="attachment_file"&gt;File&lt;/label&gt;
      &lt;input class="file optional" type="file" name="attachment[file]" id="attachment_file"&gt;&lt;/div&gt;
  
    ...
  
  &lt;/form&gt;
  </code></pre>
  
  <p>It adds the <code>file</code> class to the <code>label</code> and <code>input</code> fields. Is there a way to remove the <code>file</code> class when the form is being built?</p>
  
  <p>I'm trying to use Bootstrap 4 (Alpha) and it's clashing with the <code>file</code> class name.</p>
  
  <p>I thought I could do it on the <code>config.wrappers</code> but it adds the type of input as a class eg. <code>string, file</code>.</p>
  
  <p>Thanks in advance.</p>