What's causing this file name extension problem with Open Office?

Whenever I create a new document in O.O., I always save it as a Word document, which should give it the ’.doc’ file extension (and does, according to my files). I often share these docs with others via email attachment. Every once in a while, and for no apparent reason, the recipient can’t open the attachment.

Today, I sent two files, created on the same day, as attachments on one email. One of them showed up with .doc, followed by (application/msword) and the other without .doc, followed by (application/octet-stream). The first could be opened, the second couldn’t.

I’ve triple checked that both were saved correctly on my end, and have even tried clicking on the ‘automatic file extension’ box, but nothing is fixing it.

What have I done wrong, and how can I correct it before I go insane? I know, it may be too late for that…