title: Shrine Image uploader not mounted correctly
body: <p>I am attempting to follow this tutorial <a href="https://gorails.com/forum/direct-file-uploads-to-s3-part-2-example-gorails" rel="nofollow noreferrer">https://gorails.com/forum/direct-file-uploads-to-s3-part-2-example-gorails</a> and upon loading my local server, it spits out this error:</p>
  
  <pre><code> routing/mapper.rb:613:in `mount': A rack application must be specified (ArgumentError)
  </code></pre>
  
  <p>Here are my routes:</p>
  
  <pre><code>Rails.application.routes.draw do
   root to: "photos#index"
   resources :photos
  
   mount ImageUploader::UploadEndpoint, at: "/images/upload"
  end
  </code></pre>
  
  <p>And in case someone needs it, my shrine.rb initializer</p>
  
  <pre><code>require "shrine/storage/s3"
  
   s3_options = {
    access_key_id: "MY_ACCESS_KEY",
    secret_access_key: "MY_SECRET_KEY",
    region: "S3_REGION",
    bucket: "S3_BUCKET",
  }
  
  Shrine.storages = {
    cache: Shrine::Storage::S3.new(prefix: "cache", **s3_options),
    store: Shrine::Storage::S3.new(prefix: "store", **s3_options),
  }
  
  Shrine.plugin :activerecord
  Shrine.plugin :upload_endpoint
  Shrine.plugin :presign_endpoint
  Shrine.plugin :restore_cached_data
  </code></pre>
  
  <p>Any and all help would be greatly appreciated!</p>