MacOS – Change default app for file extension

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.

That is possible with Folder Action.

Means your files have to be in separate folders.

I assume you mean Folders when you say Directory.

As step one lets create a Automator flow for Folder Action.

Step 2 select the folder you want and create the work flow by dragging following in to it. You are done with this step, save it under a name you want for future references.

Step 3: In finder right click on the folder you want this to work on and select Services- Folder Action Setup..

In the pop up window select the file you just created. (yourfile.workflow)

That's it, now all files inside that folder will be opened with specified application, so you can now set that for other folders using the above procedure to set other applications to open same file type. It will do it automagicaly from now on as soon you drop a new file in the folder.