在Rails 6中使用activestorage时，如何在重新显示表单时保留文件？

<%= form_with(model: @user, local: true) do |form| %>
  <%= form.file_field :avatar %>
  <%= form.hidden_field :avatar_cache %>
  <%= form.submit %>
<% end %>

def new
  @user = User.new
end

def create
  @user = User.new(user_params)
  if @user.save
    # 保存成功后，将已上传文件的标识符赋值给隐藏字段
    @user.avatar.cache_id = @user.avatar.blob.id
    redirect_to @user
  else
    render 'new'
  end
end

class User < ApplicationRecord
  has_one_attached :avatar

  before_save do
    if avatar_cache.present?
      avatar.blob = ActiveStorage::Blob.find_by(id: avatar_cache)
    end
  end
end