Replace Icons by File Type on Monterey (Mac Mini M1)

I found a way to add a custom icon to all files of a particular type. This won't assign that icon to new files, though.

Start a search in Finder for files of the particular type (e.g., weblog).

Copy an image you want to use for the icon.

Select all of the files produced in the search and Show Inspector (cmd-opt-i).

Paste the image onto the icon in the icon well of the Inspector window.

Be careful when you use the command to open the inspector, if you have hundreds of files selected and you just type cmd-i, it will open Get Info for each of the files.

The associated icons are maintained by the app that is set to open the files (as P. Phillips indicated in his post).

To change all of the icons of all the files of that type, you would have to edit the application Info.plist and change the icon used for those file types. Then, you would probably have to rebuild the Launch Services database to reflect the changes you made.

Default OS Apps cannot be modified, so you would only be able to change icons for apps that you have installed separately from the OS pre-installed apps.