correct.rb

  def create
    blob = ActiveStorage::Blob.create_after_upload!(
          io: params[:file][0],
          filename: params[:file][0].original_filename,
          content_type: params[:file][0].content_type
        )

    respond_to do |format|
      format.json do
        render json: {
          "file": {
            "thumb": "",
            "url": url_for(blob),
            "id": blob.id,
            "title": blob.filename
          }
        }.to_json
      end
    end
  end

incorrect.rb

  def upload_images
    @post.images.attach(params[:file])
    respond_to do |format|
      format.json do
        render json: {
          "file": {
            "thumb": "",
            "url": url_for(@post.images.last),
            "id": @post.images.last.id,
            "title": @post.images.last.id
          }
        }.to_json
      end
    end
  end