Someone send me file with extension (filename.docx).

This is Microsoft Office 2007 Word File, or you can call your friend to save the file as 97-2003 version, i.e. .doc. However, you should make sure the file is virus free, otherwise, don't open any file you are unfamiliar via email.

This is a word file created under Office 2007, just open it by Word 2007 (under VISTA environment).