Have you checked out Mail? That is what the new ActionMailer API in Rails 3 is built upon.
"Mail is an internet library for Ruby that is designed to handle emails generation, parsing and sending in a simple, rubyesque manner."
Here comes a quick example from the docs:
require 'mail'
@mail = Mail.new
file_data = File.read('path/to/myfile.pdf')
@mail.attachments['myfile.pdf'] = { :mime_type => 'application/x-pdf',
:content => File.read('path/to/myfile.pdf') }
Update: Or even more simply:
@mail = Mail.new
@mail.add_file("/path/to/file.jpg")