I recently installed a Wine application which associated itself to the .dat extension. I tried uninstalling the application to get rid of the association, but the uninstaller did not remove it.
The next thing I tried was: 1. Open Nautilus 2. Right click on a .dat file, Properties 3. Click on the Open With tab and click Remove.
I could unset the default application, but the "file type" and the icon was still not right.
Then I found out the directory ~/.local/share/mime/application. I removed the file corresponding to x-wine-extension-dat.xml and I executed update-mime-database ~/.local/share/mime/.
I don't know how, but I believe that wine can recreate the removed file based on the information stored in the Wine Windows registry.
Therefore, I executed wine ~/.wine/drive_c/windows/regedit.exe I looked for .dat and similar associations at HKEY_CLASSES_ROOT and I deleted those keys.
Finally I think that the problem is solved.