app/views/documentos/_form.html.erb
<%= simple_form_for(:documentos) do |f| %>
<%= f.error_notification %>
<div class="form-inputs">
<%= f.input :descripcion %>
<%= file_field_tag :archivo, type: :file, multiple: false %>
</div>
<div class="form-actions">
<%= f.button :submit %>
</div>
<% end %>