Assuming we have a database project called MyDatabase then a file called MyDatabase.jfm appears in the root of the project directory.
MyDatabase
MyDatabase.jfm
The takeaway from others is...
*.jfm
.gitignore
(In powershell, Add-Content .\.gitignore *.jfm)
Add-Content .\.gitignore *.jfm
git rm --cached *.jfm