Simply save the below Reg File to a .reg extension named text file and then run the elevated command of ASSOC .resource=. Afterwards, double click the Reg File to remove that registry key and its content from the registry to fully disassociate the file extension from opening with any apps.

Reg File

Windows Registry Editor Version 5.00

[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.resource]

Now when you go to open the .resource file, it will no longer have any application association and prompt you to tell it what app to open with.

You can delete this key manually by navigating to it via regedit and then pressing Delete too.

Additional Resources

• How to add, modify, or delete registry subkeys and values by using a .reg file

• How to associate a default program during application setup

• what purpose does HKCU\software\microsoft\windows\currentversion\explorer\fileexts serve?

• ASSOC /?

  Displays or modifies file extension associations
  
  ASSOC [.ext[=[fileType]]]
  
    .ext      Specifies the file extension to associate the file type with
    fileType  Specifies the file type to associate with the file extension
  
  Type ASSOC without parameters to display the current file associations.
  If ASSOC is invoked with just a file extension, it displays the current
  file association for that file extension.  Specify nothing for the file
  type and the command will delete the association for the file extension.