title: how to add an upload button to active admin
body: <p>I am trying to add a button which will call a popup windows to upload a file on a server. I am looking for paperclip alternative without need to use image preprocessor. </p>
  
  <p>User flow is going to be next:
  - Log in into active admin
  - Click create new photo
  - Pick photo using file picker (a place where I am stuck)
  - Upload selected picture onto cloudinary or something similar to it.</p>
  
  <p>I really could use paperclip but I am afraid it will require to many other dependencies on production server. What would you advice, my friend?</p>