There is no attachment, but there is a displayed image. Yes they look like wingdings but it looks more like bit level data. There could be corruption issues here.

Is it just one file? Many files? Do all files after 10-30 minutes go this way?

It's happening on multiple files on different machines all running Office 2010 with encryption.
The timeline between incidents is pretty random, I'm starting to suspect that the encryption could be at fault here..

cya

Craig