How to Identify Which Plug In is Missing?

Question

Years ago (back in 2007), we were given some Indesign files as reference for a client's work.  We never needed to open them and, if I recall correctly, we tried but had a problem.  I believe we got an error saying that a special plug in was required.
Now we definitely are getting an error like that.  Upon opening it, we get an error that says, "Adobe InDesign may not support the file format, a plug-in that supports the file format may be missing, or the file may be open in another application."  The file format should be supported.  And the file is not open.  Is there any way to tell what plug-in might be missing?
BTW, I'm still working in CS6
Thanks!

Answer 1

Thanks for the tip.
I opened one of the failing files and an openable one in BBEdit.  They look nothing alike.  Below is the failing one. 
Still, this is weird.  The file has an indd extension.  And, it looks like it was packaged by InDesign.  It's in a folder with a Links folder and a Fonts folder.  Could it be so old that it's unopenable by CS6?
Thanks!
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