MacOS – Modify “open with” list for file type (not for duplicate removal)

There is not a safe and advisable way to achieve what you are after because this function of the OS is controlled by the Launch Services database. Developers register the filetypes their applications support with the OS on first launch.

The best way to get this fixed is to petition the developer to add support for the file extension in a future version of their application.

With that said you have already found the only documented way that Apple offers for overriding the edicts of the Launch Services database. If you want to dig deeper and take the road less traveled supported here are a few pointers.

There is a .plist file that governs part of what Launch Services does located at: ~/Library/Preferences/com.apple.LaunchServices.plist

Apple notes that removing this plist can clear out unwanted Open With contextual menu options. But that's the opposite of what you're after.

It's a binary .plist but you could convert it to an xml version and experiment a little to see if you can come up with a recipe to achieve what you're after. This isn't advisable, it could cause harm, and it will most likely get overwritten by the OS at some point.

There is also a tool hidden a bit deeper in the OS called lsregister which could be helpful in exploring Launch Services's plumbing. Here is a helpful explanation of lsregister and its options.

Applications don't have to live in /Applications. If you have a clone of your system mounted, that can account for some duplicates, but certainly not what you are observing. The list of programs for Open with… is maintained in the Launch Services database. If the list contains many duplicates as yours does, the database looks like it needs rebuilding.

Macworld Hints offers a suggestion as to how you can fix this:

The lsregister command, found at System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister, can tell the database to reset. Macworld recommends running the command thusly: /System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user

Note that utilities like Onyx or Cocktail can also reset this database for you. Do note that Finder won't re-read the new database so you will either have to log out and in again or issue a killall Finder if you don't mind just killing it for any users on your Mac to avoid a log out and in.