I too am having this problem. It first reared it's head with pdf's. I'm using windows 7 and it opens PDFs with Adobe reader as it's default program. However Outlook was using a completely different reader called Complete PDF which doesn't really cut it. So I uninstalled it and thats where my trouble began.
After uninstalling Complete PDF all mails with PDFs attached were now listed as unknown files. Even when I'm sending a mail and browse to it with the Insert File box again I'm faced with a blank icon and unknown file. However if I browse to it with windows explorer they are still associated with Adobe.
While using the Insert File box I found you could right click on the file you wanted to attach and you are presented with the Open-With option. However Outlook doesn't list it as an option and if you click choose default program again it doesn't include Adobe as a recommended option.
If you click the drop down arrow for other programs thats found below this list again Adobe isn't listed.
So naturally you would click the browse button to add Adobe to this list but when you click on the Adobe exe file nothing happens. It's not added to the recommended programs nor does it become the default program.
This is getting very frustrating. Can anyone offer some solutions