Paperclip

Install and read https://github.com/thoughtbot/paperclip and https://github.com/thoughtbot/paperclip/wiki/Thumbnail-Generation first.

$ rails generate paperclip product asset

Automatically detected.

Considering a has_attached_file :asset declaration, all :asset_file_name, :asset_content_type.. columns will be hidden. A file upload field will be created and accessible for customization in the DSL: field :asset, :paperclip.

One thing you may need is a delete method in your model. Paperclip does not include it, you'll need to add it manually. RailsAdmin will detect it and add a checkbox.

class Product < ActiveRecord::Base
  has_attached_file :asset,
    :styles => {
      :thumb => "100x100#",
      :small  => "150x150>",
      :medium => "200x200" }
  validates_attachment_content_type :asset, :content_type => /\Aimage\/.*\Z/
  # add a delete_<asset_name> method: 
  attr_accessor :delete_asset
  before_validation { self.asset.clear if self.delete_asset == '1' }
end

If you use a attr_accessible strategy, don't forget to add delete_asset to the whitelist.

More here