Many people think that MIME type for .docx documents (MS Word 2007 ) is same as .doc documents,(application/msword), but that's a blunder, because microsoft has introduced new MIME types for new version of MS Word ...
so to ease your pain of searching it here and there, i am providing valid MIME type for .docx
application/vnd.openxmlformats-officedocument.wordprocessingml.document
hope it helps and hence saves your precious time....
