ActiveAdmin+Carrierwave

base.scss

/**
 * FileInput < Formtastic::Inputs::FileInput
 */
.attachment_wrap {
    @extend p.inline-hints;
    font-style: default;
    display: block;

    &.attachment_img img {
        height: 80px;
    }

    &.attachment_plain {
    }

    [name*="[remove_"] {
        display: inline-block;
        margin-left: 5px;
    }
}

display_helper.rb

#
# https://github.com/activeadmin/activeadmin/blob/master/lib/active_admin/view_helpers/display_helper.rb
#
module ActiveAdmin
  module ViewHelpers
    module DisplayHelper

      def pretty_format_with_pretty_other_format(object)
        pretty_uploader_format(object) ||
          pretty_jsonb_format(object) ||
          pretty_format_without_pretty_other_format(object)
      end
      alias_method_chain :pretty_format, :pretty_other_format

      def pretty_uploader_format(object)
        if object.class.ancestors.include?(CarrierWave::Uploader::Base)
          if object.present?
            if object.version_exists?(:thumb) && object.thumb.file.exists?
              content_tag(:p,
                image_tag(object.url(:thumb)),
                {class: 'attachment_wrap attachment_img'}
              )
            else
              link_to object.file.identifier, object.url
            end
          end
        end
      end

      def pretty_jsonb_format(object)
        if object.class.ancestors.include?(Hash)
          content_tag(:pre,
            JSON.pretty_generate(object).gsub(":", " =>")
          )
        end
      end

    end
  end
end

file_input.rb

class FileInput < Formtastic::Inputs::FileInput
  def image_html_options
    {:class => 'attachment_wrap attachment_img'}.merge(options[:image_html] || {})
  end
  def image_plain_html_options
    {:class => 'attachment_wrap attachment_plain'}.merge(options[:image_html] || {})
  end

  def to_html
    input_wrapping do
      label_html <<
      builder.file_field(method, input_html_options) <<
      image_html
    end
  end

  protected

    def image_html
      return "".html_safe if builder.object.new_record?

      case options[:image]
      when Symbol
        builder.object.send(options[:image])
      when Proc
        options[:image].call(builder.object)
      when String
        options[:image].to_s
      else
        att = builder.object.public_send(method)
        if att.present?
          if att.version_exists?(:thumb) && att.thumb.file.exists?
            builder.template.content_tag :div, nil, image_html_options, true do
              builder.template.image_tag(att.url(:thumb) ) +
              builder.check_box("remove_#{method.to_s}".to_sym, {}, true, false)
            end
          else
            builder.template.content_tag :div, nil, image_plain_html_options, true do
              builder.template.content_tag(:span, att.url() ) +
              builder.check_box("remove_#{method.to_s}".to_sym, {}, true, false)
            end
          end
        else
          "".html_safe
        end
      end

    end
end