gistfile1.rb

# Scenario 1: remote resource returns a binary file;
# the last part of the uri represents the file name
# e.g. http://someurl.com/artists/jeanlucponty/tracks/elephants-in-love.mp3
class Audio < ActiveRecord::Base
  has_attached_file :file

  def file_from_url(url)
    self.file = download_remote_file(url)
  end

  private
  
  def download_remote_file(url)
    # OpenURI extends Kernel.open to handle URLs as files
    io = open(url)
    
    # overrides Paperclip::Upfile#original_filename;
    # we are creating a singleton method on specific object ('io')
    def io.original_filename
      base_uri.path.split('/').last
    end
    
    io.original_filename.blank? ? nil : io
  end
end

# Scenario 2: remote resource returns a binary file in the response body;
# along with the filename in the response headers, e.g.
#   Content-Type: audio/mp3
#   Content-Transfer-Encoding: binary
#   Content-Disposition: attachment; filename=chopping_broccoli.mp3"
class Audio < ActiveRecord::Base
  has_attached_file :file

  def file_from_url(url)
    self.file = download_remote_file(url)
  end

  private
  
  def download_remote_file(url)
    # OpenURI extends Kernel.open to handle URLs as files
    io = open(url)
    
    # overrides Paperclip::Upfile#original_filename;
    # we are creating a singleton method on specific object ('io')
    def io.original_filename
      # OpenURI::Meta's meta attribute returns a hash of headers
      meta["content-disposition"].match(/filename=(.+[^;])/)[1]
    end
    
    io.original_filename.blank? ? nil : io
  end
end

# example usage
audio = Audio.new
audio.content_from_url('http://someurl.com/someresource.mp3')
artist.audios << audio