File model tried to call instance on a symbol (Paperclip 3.4?)

Raises exception when uploading a file to S3: `undefined method 'instance' for :original:Symbol`.

I couldn't figure out why it was happening exactly (tried to trace it with pry-stack_explorer).

Here's what I added to my `config/initializers/comfortable_mexican_sofa.rb` to get S3 working:
```ruby
  config.upload_file_options = {
    storage: :s3,
    s3_credentials: {
      bucket:              ENV['AWS_BUCKET'],
      access_key_id:       ENV['AWS_KEY'],
      secret_access_key:   ENV['AWS_SECRET']
    }
  }
```

Checking `if f.respond_to? :instance` fixes it.

All tests passing.