What does Mail do when it sends Windows friendly attachments?
According to http://www.ncmug.org/tips/mail_attachments.html:


  By default, Mail assumes your recipient is also a Mac user and therefore includes the resource forks (if any) of attached files. Normally a Mac user sees such attachments as a single file, whereas a Windows user sees two individual files - one containing the data fork of the file and the other containing the resource fork.
  
  When you choose "Windows Friendly" attachments, Mail strips the resource fork so that Windows users receive just one file, not two (one of which would be unusable anyway). In most cases - at least for files created with modern applications - all the crucial parts of files are in the data fork; as long as the filename has the correct extension and they have an appropriate application, Windows users can open the file.