Nested form and twitter bootstrap css.

_document_fields.html.slim

- if f.object.file?
  = f.input :file, label: false, wrapper: :append do
    = f.input_field :file, as: :string, readonly: true
    span
      = link_to 'Download', f.object.file_url, class: 'btn'
      = f.link_to_remove 'Remove document', class: 'btn btn-danger'
- else
  = f.input :file, label: false, wrapper: :append do
    = f.input_field :file, as: :file
    span
      = f.link_to_remove 'Remove document', class: 'btn btn-danger'

_form.html.slim

= simple_nested_form_for(@vehicle, html: {class: 'form-inline'}) do |f|
  = f.fields_for :documents
  = f.link_to_add "Add a document", :documents