rails_website_template/app/uploaders/cover_uploader.rb

# encoding: utf-8

class CoverUploader < CarrierWave::Uploader::Base

  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  include CarrierWave::MiniMagick
  include CarrierWave::MimeTypes

  # Choose what kind of storage to use for this uploader:
  
  if Rails.env.test? or Rails.env.cucumber?
      storage :file
  end

  if Rails.env.development?
      storage :file
  end

  if Rails.env.production?
      # Include the Sprockets helpers for Rails 3.1+ asset pipeline compatibility if using fog: 
      include Sprockets::Rails::Helper
      include Sprockets::Helpers::RailsHelper  
      include Sprockets::Helpers::IsolatedHelper      
      storage :fog
  end
  
  process :set_content_type

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{mounted_as}/#{model.id}"
  end
  
  def cache_dir
    " ./tmp/uploads/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  # def default_url
  #   # For Rails 3.1+ asset pipeline compatibility:
  #   # ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.png"].compact.join('_'))
  #
  #   "/images/fallback/" + [version_name, "default.png"].compact.join('_')
  # end

  # Process files as they are uploaded:
  # process :scale => [200, 300]
  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  version :thumb do
     process :resize_to_fit => [300, 200]
  end
  
  version :mini do
     process :resize_to_fit => [150, 100]
  end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  # def extension_white_list
  #   %w(jpg jpeg gif png)
  # end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
  # def filename
  #   "something.jpg" if original_filename
  # end

end