# given our `post` example, param_key will be `post`
file = params.nested_file?(:param_key)["image_attribute"]?
if file
  # file.name
  # "param_key:image_attribute"
  # file.tempfile
  #<File:/var/folders/41/g51hx5q90xqgk7wz9qv6738c0000gn/T/.3uQb3Bparam_key:image_attribute (closed)>

  SaveFileImport.create!(upload: file, post_id: post.id)
end