Editing a Microsoft Office file via. WebDAV will change the file type in Blackboard
When your students click on the attachment in your course to view the file, they will either be greeted with an error or, in the case of Internet Explorer, a white page.
At the time of writing (Feb 2016), Blackboard have no target for when this will be fixed.
The technical details
When you edit your Office file, Blackboard/WebDAV is changing the MIME type to
text/xml. When your students click on the link to download the file, Blackboard tells their web browsers that the file is XML. Their web browsers tries to render the file as XML and it fails.