Only fix for 3.4.2 involves hacking a core file. The issue is already corrected in the upcoming version 3.5.
Edit wp-includes/functions.php. Around line 1810, you’ll find this:
'doc|docx' => 'application/msword',
Change that to this:
'doc' => 'application/msword',
The docx file extension is actually already handled properly later down in the file, line 1819:
'docx|dotx' => 'application/vnd.openxmlformats-officedocument.wordprocessingml',
So it will work properly once you remove the spurious extension.
Again, hacking core files is a bad thing, but seeing as the refactorization in WordPress 3.5 corrects the issue already, and there’s no way to do this in a plugin or theme or even with a proper drop-in, I think that this specific case might justify it.