Image Upload

Hi,

I have an application with the models Section and Element with polymorphic associations to Paragraph and Image.

```
class Section < ActiveRecord::Base
has_many :elements
end
```

```
class Element < ActiveRecord::Base
belongs_to :section
belongs_to :implementation, polymorphic: true
accepts_nested_attributes_for :implementation
end
```

```
class Paragraph < ActiveRecord::Base
has_one :element, as: :implementation
end
```

```
class Image < ActiveRecord::Base
mount_uploader :file, ImageUploader
has_one :element, as: :implementation
end
```

I was able to make the Section form with the Paragraph class. However, with Image class I not able to upload files due to file fields in form are stripped out as you mentioned in the to documentation.

I have already configured carrierwave + remotipart and actually images are uploaded asynchronously from the new image form (outsite the Section form). However, but I was not able to make that work within the Section form. I have noticed that file are still stripped out only when I try to add and send a new Image from the Section form.

Do you have an example or more detailed documentation regarding image uploading?

Thanks in advance

该 issue 提到了 Section、Element、Paragraph、Image、CarrierWave、Remotipart 和 Section 表单；首先在其中复现嵌套创建 Image 的过程，并将其与独立的新建 Image 表单进行比较。完成的标准是，有示例或文档说明所需的设置，并且可以从 Section 表单上传图片。

由索引模型根据 Issue 内容生成。