I moved a lost sequence out of Avid Attic into the Avid project, worked on it all morning, closed it without closing the project, and now it appears as a Document instead of an AVID Bin File as it was when I was working on it. And it no longer appears in my project. I have worked on this project on my Raid drive for over a year without this trouble. I just noticed that Avid said not to upgrade to El Capitan but I did. Could that be it?
Can't say for Mac, but on Windows the bin files saved in the Attic typically have no extension. I always have to add the .avb to the filename once I copy it out of the Attic. Your OS could be trying to help you out by automatically assigning it a document extension.