I obtained .1 bitcoin ( I believe that was the amount) back in either 2011 or 2013 ( I have BC files on my computer from both of those dates ). I wanted to get involved in using them so I downloaded the current wallet and ran the synchronization. I have what I believe are all the old files on a remote hard drive. A search reveals 89 files ( none of them dat ). They are H, RC ,QM, CPP, application, PNG, ICNS.
A quick google search of these extensions tells me that they are just from the source code.
Other than the application which will be the runnable application.
I think the file has always been .dat since the creation of bitcoin core so you may not have your 0.1
BTC.
ICNS and JPG are image files.
H, RC, QM and CPP are all programming code languages/extensions.