I wanted to make .rel files open in Wordpad and .rdb files in Rel.exe (not that I actually expect to be double-clicking on .rdb files). It turns out to be extraordinarily difficult to make the required associations in Windows 10 compared with previous Windows version I have used (Vista was the most recent).
I always used to use "Open with" on the context menu and check "Always use this program ..." so that straightforward double-click will work in future. In Windows 10 that "Always use this program" option doesn't always appear and you have to fall back on Control Panel/Default Programs/Associate file type ... with a program. But that works from a huge list of file types (extensions) from which you have to select the one you wish to deal with, and the one you want won't necessarily appear! I had to do a web search to find out how to make .rel and .rdb appear in this list. It's about as counterintuitve as you can get:
1. Control Panel
2. Indexing Options (obviously?
)3. Advanced
4. Move to "File Types" tab
5. Enter new extension in box provided
6. Click the "Add" button
Hugh