Legacy Database CarrierWave Integration

Question

I am having legacy database
Rails 3.2 & ruby 1.9.3p448
I wanted to develop active_admin back-end application
I wanted use CarrierWave File Uploads gem for uploading image
Following is my code in admin/item_master.rb i.e in view

  form :html => { :enctype => "multipart/form-data" } do |f|

    f.inputs "Item Details" do
      f.input :IMTNAME
      f.input :IMTBRIEFDESC
      f.input :IMTDETAILDESC
      f.input :IMTIMAGE, :as => :file
    end
    f.actions
  end

Following is my code in model

class ItemMaster < ActiveRecord::Base
  set_table_name "MDIMT"

  attr_accessible :IMTNAME, :IMTBRIEFDESC, :IMTDETAILDESC, :IMTIMAGE

  mount_uploader :IMTIMAGE, ImtimageUploader
end

It givesme Following Error

NameError in Admin::ItemMastersController#update
uninitialized constant IMTIMAGE