I've got a user looking to store .oft files in a SharePoint Online document library, and they were expecting to be able to open them inline. Instead the only option is to save the file. I'm guessing that it's because the .oft extension doesn't have a mapped MIME type, and so it's falling foul of the 'X-Download-Options' header.
If this was on-premise, I'd be either looking to add the MIME type in to the IIS server (best case), or setting Permissive mode on the farm. As I can't get that deep into the SPO stack, is there any alternative here, or should we go back to the drawing board?