Hello and welcome to TSF
this solution will have you to edit the registry so before doing anything, first back up the registry, to do so:
press start, in the search box type:
and press enter
now click on file -> Export, choose where to save and save it, this will backup your registry, now if anything bad happens after you try what I'll tell you next, just go to regedit again, click import and choose where you saved the registry to import it.
Now what you need to do:
Go to Regedit, on the left panel there should be some file extensions
now to access one of those, double click on it so every "->" in the exemple is a double click, here we go:
-> HKEY_CURRENT_USER -> Software -> Microsoft -> Windows -> CurrentVersion -> Explorer -> FileExts
now you are in the FileExts, you can see lots and lots of extensions (eg: .exe, .txt, etc....)
search for .dat
double click on it, and delete "OpenWithList" (right click, Delete)
Close Regedit and restart your PC
!!Please do not PM me about Tech Support!!