CarrierWave extension-aware remote file downloading

avatar_uploader.rb

class AvatarUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick

  def extension_white_list
    model.skip_avatar_extension_check ? nil : %w(jpg jpeg gif png bmp)
  end
end

profile.rb

class Profile < ActiveRecord::Base
   mount_uploader :avatar, AvatarUploader
   attr_accessor :skip_avatar_extension_check

   def file_avatar_url=(url)
    begin
      self.skip_avatar_extension_check = true
      avatar.download!(url)
    ensure
      self.skip_avatar_extension_check = false
    end

    path = avatar.path
    ext = File.extname(path)

    logger.debug "Downloaded file extension - #{ext}"

    if ext.blank? && ext = detect_extension(path)
      logger.debug "Detected extension for avatar - #{ext}"
      path = path + ".#{ext}"
      FileUtils.mv(avatar.path, path)
      avatar.remove!
      self.avatar = File.open(path)
    end

    avatar.send(:check_whitelist!, CarrierWave::SanitizedFile.new(File.open(path)))

    avatar.store!
    save!
  end

  protected

  def detect_extension(file_name)
    mime_type = %x(file --mime-type #{file_name}|cut -f2 -d' ').gsub("\n", "")
    case mime_type
    when 'image/jpeg'
      'jpg'
    when 'image/jpg'
      'jpg'
    when 'image/png'
      'png'
    when 'image/gif'
      'gif'
    end
  end
end

A little simplier:

mime_type = %x(file --mime-type -b #{file_name}).gsub("\n", '')