Create a custom execute that modifies associated file types after you install a package.
Use the Windows Assoc command tool to determine if a file extension association already exists, and return the file type name. Define the file type with Assoc if the extension does not have any existing file type association.
Use the Windows Ftype command tool to assign the application associated with the file type.
Replace <FileType> with the file type you want to modify, and <Path> with the full path of the executable you want to associate the file type with.