People will receive attached files that you send just as they always have.
How others receive attachments that you send has not changed. What has changed is the file format used by some 2007 release programs such as Word 2007 (now .docx), PowerPoint 2007 (now .pptx), Excel 2007 (now .xlsx), and Access 2007 (now .accdb).
This new format keeps file sizes smaller and helps keep files safer. Attachments that you send or receive may use this new format. Outlook fully supports sending and receiving files that use the new formats.