how to use Paperclip alone with a separated attachment model

I have a scenario that my user need to upload a image first and then
assign to another model, so I used a generic model, to upload the file
first. But I always get a exception of
ActiveRecord::UnknownAttributeError (unknown attribute: utf8). because
rails automatically adds a utf-8 field for the form submit.

model:
<code>
class AttachedImage < ActiveRecord::Base
  belongs_to :attachable, :polymorphic => true

  has_attached_file :image
end
</code>

migration:
<code>
class CreateAttachedImages < ActiveRecord::Migration
  def change
    create_table :attached_images do |t|

      t.string :image_file_name
      t.string :image_content_type
      t.integer :image_file_size
      t.datetime :image_updated_at
      t.references :attachable, :polymorphic => true

      t.timestamps
    end
  end
end
</code>

And the controller:
<code>
class AttachedImagesController < ApplicationController
  def create

    @attched_image = AttachedImage.new(params)

    respond_to do |format|
      if @attched_image.save
        format.js
      else
        format.js
      end
    end
  end
end
</code>

the view part:
<code>
<div id="upload-image-dialog">
    <%= form_tag(@attached_image, :method => "POST", :remote => true,
:html => { :multipart => true }) do %>
      <%= file_field_tag :image %>
      <%= submit_tag("submit") %>
    <% end %>
    <h1 style="display:none">Successfully Uploaded.</h1>
</div>
</code>

it's quite straight forward, but every time I submit this form, I will
get an exception of the utf8 field added automatically by rails.

ActiveRecord::UnknownAttributeError (unknown attribute: utf8):
  app/controllers/attached_images_controller.rb:5:in `new'
  app/controllers/attached_images_controller.rb:5:in `create'

I could not understand that, this shall not be a problem right? We are
writing @model = Model.new(params) everyday, hope someone could help me
out and explain what's going on under the hood, thanks!

Problem is here, in this line of code:

@attched_image = AttachedImage.new(params)

It should be:

@attched_image = AttachedImage.new(params[:attached_image])

change in the controller

@attched_image = AttachedImage.new(params)

to

@attched_image = AttachedImage.new(params[:attached_image])

if above not working because of form_tag usage

then you have to use custom new like

@attched_image = AttachedImage.new(:image=>params[:image])

this should work.