How do I revert a change I made to a specific file type's default application?
I wouldn‘t be to worried about the .dat files. They are only 64 bytes in size, so there can‘t really be much inside of them that could cause harm. Have you tried to open it with a text editor and see what’s inside?

If you don‘t trust it, uninstall it and the .mkv apps should no longer have a default application to open with. Alternatively, just change the default app to QuickTimePlayer.

Another option is to use a third party app called RCDefaultApp. With it, in the Extensions tab, you can choose your file type and choose an application for it. With the „disable“ option you can set it to not having a standard application to open with (see screenshots on website).