Invalid document file format - error
Thank you for adding the screenshot - it makes the issue much clearer.  I actually haven't used this feature before and it seems to be undocumented :(  That said, I was able to use it successfully on my local install.

While this might be a bug you're seeing, I looked at the code and am suspecting a MIME misconfiguration in the website software settings.  To help track down which it is, I recommend editing the file at &lt;civiroot&gt;/CRM/Admin/Form/MessageTemplates.php.

Find this line:
$errors['file_id'] = ts('Invalid document file format');

Replace it with:
$errors['file_id'] = 'MIME type: ' . $files['file_id']['type'];

The acceptable values are:


docx: application/vnd.openxmlformats-officedocument.wordprocessingml.document
odt: application/vnd.oasis.opendocument.text


If you're getting a different value, hopefully it will provide a helpful clue as to the problem.  If not, please edit the question again to show the error you're getting.