When you save a file, excel will save it as a temporary file (8 characters, no
extension). Then if that save is successful, excel will delete the original and
rename this funny named file to the original's name.
You see the results when something interferes with this process.
A couple of things that get pointed at are antivirus software and network
problems (hardware or permissions).
Maybe you could disable the antivirus software (temporarily) to see if that
helped.
If the problem goes away, then visit your local AV site and see if they have
updates. If it doesn't, I'd think I'd try to get my IT department more
involved. (well, unless you're already the IT department!)
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I'm guessing that the extension doesn't actually become File, but the file type
in windows|explorer shows File (instead of something like "text document" or
"microsoft excel worksheet".