Some advice on creating attachments might also be useful here. Try to create your attachment in a format you know a large number of people will be able to access. Word is good for most businesses, while WordPerfect is ideal only if you are sending your resume to a legal firm. Try to avoid creating and attaching your resume as MS Works files or Wordstar, etc. You may use these programs to create basic .txt or .doc files though, which anyone should be able to view.
Some people attach their resumes as HTML files.
This has the advantage of being almost as universal as a text or document file because most businesses have a browser on their workstations. HTML files are restricted in their formatting though and do not always copy nicely to other text based programs, especially those used as resume databases by companies. Do not expect an employer to do your hard work for you. Not all employers have easy access to the web. Many companies prohibit internal use of the web owing to productivity issues.