Since upgrading to 3.1.6 and installing the MIME validator I'm unable to upload office 2007 files via any method...

If I remove the MIME validator, uploads work as expected. I've added the following to my config file and still no luck

Injector:
  Upload_Validator:
    class: MimeUploadValidator
File:
  allowed_extensions:
    - 'docx'
HTTP:
  MimeTypes:
   'docx' : 'application/vnd.openxmlformats-officedocument.wordprocessingml.document'

Is there anything else I should be doing?

The module uses php finfo, and googling 'finfo' and 'docx ' comes up with lots of issues concerning MS 2007 files, saying that for these files the mime type is actually application/zip. Haven't checked this myself, but who knows :)

Thanks for that tip, it pointed me in the right direction. application/zip as you and many others on Stack Overflow suggested didn't work. So after running the following on some test files

<?php
$finfo = finfo_open(FILEINFO_MIME_TYPE); // return mime type ala mimetype extension
foreach (glob("*") as $filename) {
    echo finfo_file($finfo, $filename) . "\n";
}
finfo_close($finfo);
?>

The result was application/msword

Adding this to the config allows docx to be uploaded..

Great, glad I could at least be of some help :).

I saw you created an issue on https://github.com/silverstripe-labs/silverstripe-mimevalidator/issues. Maybe a good idea to share it there as well...

Agreed, but wanted to sort the other formats first. Seems xlsx isn't playing nicely regardless of what I set for it.