I just had the same thing happen to me with a spreadsheet I was updating daily. It probably is sync related I'll admit. Good news though, I was able to find my attachment by going into: TOOLS > OPTIONS. Then in the General tab there is a link at the bottom of the window "Open Database Folder". Sure enough, there's a temp folder called attachments and a backup of the file was there. Note, it was labeled" blahblah.xlsx.backup so I made a copy of the file and removed .backup. That easy, I had my file back. Not saying this will work every time since I believe these are temp folders, but it saved me. Hope it helps. I'm using a PC btw.