In another move highlighting Facebook’s strategic relationship with investor Microsoft, the company has given the established office software provider special online access to users in its new Message Inbox system.

Users who upgrade to the new version of the Inbox will see the option to view Doc attachments online — but not anything else.

Message attachments generally work as you’d expect. Click on the paperclip logo in the message creation form, then select the document on your desktop that you’d like to upload. Send it — to yourself, if you’re looking to test the integration out for yourself.

The sent message will show your attached documents. If they end in a Doc format for Word, Excel or Powerpoint, you’ll also see a dropdown that includes two links — one to download and one to see the preview on Microsoft’s Office Live site. No other types of documents will be available in preview mode.

Also, if you want to edit, save and download the attachments, you’ll need to have a desktop version of Office installed.

We talked to Facebook engineers about this integration, to see if any other formats (like PDFs) might somehow become available in preview mode. They said it wasn’t what they’re focusing on now.

So, Microsoft gets a subtle but powerful promotion in Facebook, while users can only view Docs inside Facebook, or otherwise download the document and read it that way — a less convenient choice.

Gmail and other email programs have tried to make previewing as easy as possible for all formats. For Facebook, one ramification of Microsoft’s special access is that the product offers less value for users who want to share a document in any other format. For those people, the company could one day decide to provide a more universal uploading feature.