Microsoft confirmed on November 8, 2018 that the option to set custom file associations is broken for some users on Windows 10 version 1803.
The issue, first reported on November 5, 2018 by Chris Hoffman on How To Geek, prevents users and administrators from selecting custom file association defaults.
Windows associates file types such as .txt, .pdf, or .jpg, with programs and opens files in the associated program when a user double-clicks on a file or right-clicks on a file and selects open from the context menu.
All versions of Windows use sets of default file associations for compatibility and usability. Text documents open in Notepad by default and images in the Photos application.
Users may change file associations so that files open in preferred programs instead. Text documents could be opened in Notepad++ instead or images in an image viewer that is faster than the Photos application.
Windows 10 supports two main options to change file associations in the frontend. Associations can be changed under Settings > Apps > Default Apps, or with the help of the Open With > Always use this app menu when a file type without association is loaded.