Remember....md

Active storage

• Does not save files to disk until commit.

Post.transaction do
  # This one is ok. Returns true.
  @post.attachments.attach(io: StringIO.new("something...."), filename: "new-post.txt", content_type: "plain/text")
  
  # This would raise ActiveStorage::FileNotFound error.
  # Plus, you won't find any new file in the filesystem until commit.
  # @post.attachments.last.download
end

# Meanwhile, outside the transaction (in case COMMIT), the file is available.
@post.attachments.last.download